Fix tab index

Fixes #1892

See merge request !1401

Fix comments

Fixes:

* drag-n-drop works for diff comments
* selecting images works for diff comments
* preview works for edit comments
* drag-n-drop works for edit comments
* selecting images works for edit comments
* if comment presented on 'Discussion' and 'Changes' tabs - after edit it updates on both tabs.
* Correctly hide form when edit comment on diff
* fix full-screen for diff comments

See merge request !1398

View link first

See merge request !1397

Fix scroll issue

Fixes #1886

See merge request !1396

This reverts commit 1e45ba7f.

This reverts commit 204b3c12.

Fix anchor bug

Fixes #1886

See merge request !1395

When use anchors - conetent gets hidden under navbar. This commit fixes
with hack using margin+height combination

This reverts commit b1792d9e.

This reverts commit 5140bd88.

Rescue database error

We need to check which database is setup but we cannot assume that the database exists already. Not checking this will break rake gitlab:setup for packaging as we need to do asset precompile before the database exists.

See merge request !1393

Enable signup by default

See merge request !1394

Doorkeeper update

https://gitlab.com/gitlab-org/omnibus-gitlab/issues/398

See merge request !1392

Gitlab shell bump

See merge request !1391

OAuth2 API documentation

https://dev.gitlab.org/gitlab/gitlabhq/issues/1713

See merge request !1388