Get "Merge when build succeeds" to work when commits were pushed to MR target branch while builds were running

A limited number of pages have defined their own descriptions, but
otherwise we default to the Project's description (if `@Project` is
set), or the old `brand_title` fallback.

The image will either be the uploaded project icon (never a generated
one), the user's uploaded icon or Gravatar, or, finally, the GitLab
logo.

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>

Prior, any of the specified IDs could have been hijacked by a table of
contents header, breaking the tab functionality. For example, a `##
Notes` header would get the id `notes` and prevent the Discussion tab
from being activated.

Closes #3908

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>

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>

This reverts commit 5daf44b7, reversing
changes made to 2f706fbd.

This reverts commit 4773f38e, reversing
changes made to 0d5d80b7.

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

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

On the first load, the merge request tabs would be requested in JSON format to render
the HTML. This would cause the "Reload with full diff" button to create a link to diff.json.

Closes #2041