Greatly improve external_issue_tracker performance

See 3d41133d48f6522b8755bb91b804864e8e520871 for all the details. As an aside, this commit contains a set of migrations that will introduce downtime as they add a column with a default value which in turn locks the entire table (at least on PostgreSQL).

See merge request !2498

This greatly improves the performance of Project#external_issue_tracker
by moving most of the fields queried in Ruby to the database and letting
the database handle all logic. Prior to this change the process of
finding an external issue tracker was along the lines of the following:

1. Load all project services into memory.
2. Reduce the list to only services where "issue_tracker?" returns true
3. Reduce the list from step 2 to service where "default?" returns false
4. Find the first service where "activated?" returns true

This has to two big problems:

1. Loading all services into memory only to reduce the list down to a
   single item later on is a waste of memory (and slow timing wise).
2. Calling Array#select followed by Array#reject followed by Array#find
   allocates extra objects when this really isn't needed.

To work around this the following service fields have been moved to the
database (instead of being hardcoded):

* category
* default

This in turn means we can get the external issue tracker using the
following query:

    SELECT *
    FROM services
    WHERE active IS TRUE
    AND default IS FALSE
    AND category = 'issue_tracker'
    AND project_id = XXX
    LIMIT 1

This coupled with memoizing the result (just as before this commit)
greatly reduces the time it takes for Project#external_issue_tracker to
complete. The exact reduction depends on one's environment, but locally
the execution time is reduced from roughly 230 ms to only 2 ms (= a
reduction of almost 180x).

Fixes gitlab-org/gitlab-ce#10771

Fix double request issue in artifacts browser

This fixed problem with double propagation when clicking a link that lies inside tr with clicked event attached.

See merge request !2496

Raise an error on page load when there are pending migrations in
development mode.

Fixes #5990 

See merge request !2491

Fix positioning issues with browse files button

Issue is that in Firefox clipboard is blocking the browse files
from moving all the way to the right.
Move clipboard up 1px into -1px to unblock "browse files"

SS from firefox:

![Screen_Shot_2016-01-18_at_4.01.58_PM](/uploads/312a5f1e55ca707081f718e2413416fb/Screen_Shot_2016-01-18_at_4.01.58_PM.png)

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/10849


See merge request !2489

Search title gets clipped vertically.

Fixed by adding height to search title css.

**Before:**
![Screen_Shot_2016-01-16_at_10.12.14_AM](/uploads/2651de6ae38251e1f20acade0c69a629/Screen_Shot_2016-01-16_at_10.12.14_AM.png)

**After:**
![Screen_Shot_2016-01-16_at_10.12.05_AM](/uploads/6852e9a653bde1975d7fdfa4dbc302d8/Screen_Shot_2016-01-16_at_10.12.05_AM.png)

See merge request !2460

Reduce margin to fit dropdowns

![Screen_Shot_2016-01-18_at_7.51.23_PM](/uploads/03336f86fde25d6d045742297a19751c/Screen_Shot_2016-01-18_at_7.51.23_PM.png)

Fixes #10747 

See merge request !2492

Fixes #10747

Move entry `Fix: As an admin, cannot add oneself as a member to a
group/project`

The CHANGELOG entry said it was fixed in 8.2.3. However, I found
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/1942 that
suggests it wasn't fixed until 8.3.0. A customer experienced this
issue on 8.2.3, still. This ultimately led to me investigating this
further.

[ci skip]

See merge request !2488

Fix limit for `data` column in `events` table when using MySQL

Closes #9084 

See merge request !2486

[ci skip]

Update devise to version 3.5.4

Closes https://dev.gitlab.org/gitlab/gitlabhq/issues/2642

See merge request !2490

Issue is that in Firefox clipboard is blocking the browse files
from moving all the way to the right.
Move clipboard up 1px into -1px to unblock browse files

Improve UX in build artifacts browser

This improves UX in build artifacts browser, and makes it more consistent with repository tree browser we already have.

---
Before:

![ux_artifacts_before](/uploads/9869ff74afd5e6636f0df81c7823fb6f/ux_artifacts_before.png)

---
After:

![ux_artifacts_after](/uploads/cdb68925f4489bb852fd701870e6d6a0/ux_artifacts_after.png)

---

It is easier to click on a file/directory entry now, because we don't need to click on a entry's basename (that can be really short, like `..` parent directory). We also show `Compressed to` size that indicates how much storage does this entry really consume.

Row for an entry is also highlighted on hover and we do not have Download button (clicking on a file row / file basename is enough to download a file).

Closes #10242 

See merge request !2477

Remove leading comma when user is the only one to award that emoji

Fixes #9073

See merge request !2450

Use gitlab-workhorse 0.6.0

[ci skip]

See merge request !2483

Ensure Gravatar host looks like an actual host

Solves #10243.

I've chosen to simplify the method that extracts the host: since we
only need the host, let's get rid of the path and thus get rid of the
escaping problems!

Unit tests should ensure that most of the cases are covered.

See merge request !2482

See merge request !2481

See merge request !2480

See merge request !2479

When using IssuableFinder with a Group we can greatly reduce the amount
of projects operated on (due to not including all public/internal
projects) by simply passing it down to the ProjectsFinder class.

This reduces the timings of the involved queries from roughly 300
ms to roughly 20 ms.

Fixes gitlab-org/gitlab-ce#4071, gitlab-org/gitlab-ce#3707