Fix importers with OCC

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/1180

Using validates_uniqueness_of does not guarantee the absence of duplicate record insertions. When users click button "import all" it will create duplicating group in database and data integrity will be broken.

http://api.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html#method-i-validates_uniqueness_of

I use optimistic concurrency control for handling this situation.

See merge request !1647

Reject access to group/project avatar if the user doesn't have access.

Closes #2050.

I'll add tests for EE logos once this is merged here and merged into EE.

See merge request !1671

Fix highlight of selected lines

* show color for highlighted line
* fix overflow by header

See merge request !1672

Properly move over `issues_tracker_id` from old custom issue tracker URLs.

Addresses https://gitlab.com/gitlab-org/gitlab-ce/issues/1124.

cc @marin Regression in 7.8.0.

See merge request !1670

Use Gitlab::Git helper methods and constants as much as possible.

As discussed in https://dev.gitlab.org/gitlab/gitlabhq/commit/1cc41ce1adce23d3eb4a9c4ed0ab17e11ea1f1e3#note_38702.

See merge request !1668

Mention EmailsOnPush changes in changelog.

See merge request !1667

Blocking user does not remove him/her from project/groups but show blocked label

For #248

See merge request !1664

Add criteria for requesting CVE

For #1887. Based on https://dev.gitlab.org/gitlab/gitlabhq/issues/1945#note_34587

cc @sytse

See merge request !1662

Font size 14px

Increase base font size from 13 to 14px. We already have it in most of ui elements so no need to duplicate `font-size: 14px` everywhere - just change base variable

See merge request !1663

Apply more styles from Flatly theme

Also add bottom margin for footer links on login page

See merge request !1661

Improve milestones ui

* change progress bar colors
* make milestone title bold

See merge request !1660

Also add bottom margin for footer links on login page

Add starred projects page to dashboard

For #1836 and #2069

See merge request !1659

Improve user block/unblock UI in admin area

Fixes #2111

See merge request !1658

Move profile groups page to dashboard

For #2069

See merge request !1654

Fix mass SQL statements during initial push of a big repository

## What does this MR do?
This MR prevents `process_commit_messages(ref)` to be executed for the initial push to the default branch.
Currently it is being executed which results in ~500000 SQL Statements during the import of the linux mainline repository.
These statements check for references to issues/mrs in each commit.
That usually doesn't make sense for the initial push, because there aren't any issues/mrs yet.

This MR will reduce the load on gitlab servers in the coming months. Especially now that several importers are available for gitlab and projects are moving from gitorious to gitlab.

See merge request !376