Import GitHub wiki into GitLab

Closes #2834 

GitHub doesn’t apply any constraints to theirs wiki slug allowing chars like ,, :, *, etc, we need to remove our constraints or some wiki pages will not be available after they are imported.

Some wikis use the Gollum's tags to link its internal/external resources like: images, urls, wiki pages, etc. So, to avoid that wiki links/images displays completely broken after they were imported, we added a new `WikiPipeline`, that for now will parse only simple links, and image tags.

##### Before `WikiPipeline`:

![Screenshot_2016-01-11_20.14.48](/uploads/46fd5dbb5acfc70aa8ecca3050b675e4/Screenshot_2016-01-11_20.14.48.png)

##### After `WikiPipeline`:

![Screenshot_2016-01-11_20.15.56](/uploads/b1d94aa852f385f867a7868c358b4257/Screenshot_2016-01-11_20.15.56.png)

See merge request !2324

Unsubscribe from thread through link in email footer

Closes #3437 

![Screenshot_from_2015-12-11_15-35-06](/uploads/cfb3d8737d4757f527995411f103d0ce/Screenshot_from_2015-12-11_15-35-06.png)

![Screenshot_from_2015-12-11_15-35-56](/uploads/9b7121be7ce4b05e5995ca6d38c5bea4/Screenshot_from_2015-12-11_15-35-56.png)

See merge request !2068

Standartize buttons and fields size

Fixes #8069 and #8070

See merge request !2428

Since GitHub doesn’t apply these constraints to theirs wiki slug
allowing characters like `,`, `:`, `*`, etc, we need to relax our
constraints or some wiki pages will not be available after they were
imported.

For an example the Devise project have a wiki page with the following
slug: “How To: Add sign_in, sign_out, and sign_up links to your layout
template”

fixes bug with commenting on merge requests



See merge request !2415

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

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

Add browser for build artifacts

Discussion in #3426, closes #3426.

See merge request !2123

Add variables API

References #4264 

See merge request !2264

Added also a `Gitlab::Ci::Build::Artifacts::Metadata::ParserError`
exception class.

[ci skip]

We do not want to allow runners to upload a metadata file. This needs to
be generated by Workhorse only.