Add option to mirror an upstream repository.

Closes internal https://dev.gitlab.org/gitlab/gitlab-ee/issues/279

Depends on gitlab-org/gitlab-shell!29

To do:

- [x] Decide on what user should be the author of the activity feed events. The initial project creator perhaps? That makes sense for personal projects, but less so for group projects, where it's even possible that the creator has since left the team.
- [x] Write specs!
- [x] Write documentation
- [x] Port back relevant commits to CE: gitlab-org/gitlab-ce!1822

----

## Mirror status on project homepage

![Screen_Shot_2015-11-12_at_12.48.15](/uploads/39666b0e4a14723407b0f66e2aaddb46/Screen_Shot_2015-11-12_at_12.48.15.png)


## Settings navigation item

![Screen_Shot_2015-11-10_at_17.32.27](/uploads/116ece85f65681bbd3c7c5e282ab3b3c/Screen_Shot_2015-11-10_at_17.32.27.png)

## Mirroring settings

![Screen_Shot_2015-11-17_at_11.17.17](/uploads/ff57c89831524435ec6a063c3c470203/Screen_Shot_2015-11-17_at_11.17.17.png)


## Activity feed with new "pushes" (mirror updates)
![activity_feed](/uploads/04754251f2142fd06b1fddc2aae45232/activity_feed.png)

I have yet to decide on what user should be the author of these events. The initial project creator perhaps? That makes sense for personal projects, but less so for group projects, where it's even possible that the creator has since left the team.

## Warning that branch has diverged from upstream
![diverged](/uploads/ecf7ae6c5709f626c750695f45ca521a/diverged.png)

## Commits heading after failed update
![commits_failure](/uploads/d1b3e1f6796431c839ee9e2f67499283/commits_failure.png)

Link send user to the mirroring settings if they are a project admin.

## Mirroring settings after failed update

![Screen_Shot_2015-11-12_at_12.43.09](/uploads/ece68d5021c9a4c374bf6d973cf9fafa/Screen_Shot_2015-11-12_at_12.43.09.png)

## New project page
![new_project](/uploads/296beb6c7faf390a9b7e1facc9c1d4d6/new_project.png)

## Import form after failed import
(Screenshot outdated)

![new_import_failure](/uploads/ac0ebfc8ec23232603126fe0a6e10171/new_import_failure.png)

Shown when the initial import of a project fails, not when an update of an existing project fails.

cc @dzaporozhets @sytses @JobV

See merge request !51

[ci skip]

[ci skip]

[ci skip]

[ci skip]

Added documentation on how to downgrade from EE to CE

/cc @dblessing 

See merge request !56

# Conflicts:
#	doc/README.md

CE-to-EE master

See merge request !54

Show specific runners from projects where user is master or owner

This fix for permission escalation when handling specific runners.

The users were allowed to assign runners from projects where they were guests.


See merge request !1809

Remove some code duplication

* remove duplicate code in uploaders
* remove duplicate code in NotificationHelper
* remove duplicate code in Repository
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !1800

Git LFS  support

Part of gitlab-org/gitlab-ce#2955

See merge request !1727

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Disabling cache for test environment



See merge request !1807

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Revert "Merge pull request #9812 from chrspeich/hide-tabs-lone-auth-provider"

This reverts commit 84999611, reversing
changes made to 0d9fb211.

See merge request !1805

This reverts commit 84999611, reversing
changes made to 0d9fb211.

Relative links in project README should point to default branch, not master

Relative links in the README shown on a project's main page currently always point to the "master" branch.  This is a particularly obvious problem for projects that have no "master" branch, since the link points to a non-existent file.

This patch changes the relative links to instead point to the default branch.

Resolves #2707, #1134, #1852

See merge request !1801