Make migrations reversible

/cc @jacobvosmaer @marin

See merge request !622

Two-factor authentication

Implement's Two-factor authentication using tokens.

- [X] Authentication logic
- [X] Enable/disable 2FA feature
- [x] Make 2-step login process if 2FA enabled
- [x] Backup codes
- [x] Backup code removed after being used
- [x] Check backup codes for mysql db (mention mysql limitation if applied)
- [x] Add tests
- [x] Test if https://github.com/tinfoil/devise-two-factor#disabling-automatic-login-after-password-resets applies, and address if so
- [x] Wait for fixed version of `attr_encrypted` or fork and use forked version - https://github.com/attr-encrypted/attr_encrypted/issues/155

Fixes http://feedback.gitlab.com/forums/176466-general/suggestions/4516817-implement-two-factor-authentication-2fa

See merge request !474

Bump gitlab_git to 7.1.11 to fix bug with viewing a tag on a project with submodules

This is a reminder MR to bump gitlab_git before the release. Tests will fail until this happens.

Closes https://github.com/gitlabhq/gitlabhq/issues/8411

See merge request !607

Fix commit time being displayed in the wrong timezone in some cases

This MR makes sure that the displayed commit date is shown in the timezone that's configured in the gitlab.yml.
Before it was possible that the `committed_date` of a commit would display different values on the dashboard and the commit page.

fixes gitlab-org/gitlab-ce#1278

/cc @DouweM

See merge request !624

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

Add Create Merge Request buttons to commits page and push event.

Commits page:

![Screen_Shot_2015-05-09_at_22.27.09](https://gitlab.com/gitlab-org/gitlab-ce/uploads/f77c2e47f768b2fc132de6e3a2a0323e/Screen_Shot_2015-05-09_at_22.27.09.png)

Branches page:

![Screen_Shot_2015-05-09_at_22.55.24](https://gitlab.com/gitlab-org/gitlab-ce/uploads/b0a6374f5c5d830ef19de9694e603c8f/Screen_Shot_2015-05-09_at_22.55.24.png)

Push event: (Only for new branches, and only shown to the pushing user)

![Screen_Shot_2015-05-09_at_22.27.21](https://gitlab.com/gitlab-org/gitlab-ce/uploads/989c9cb66ee03d09b96809452749d2b8/Screen_Shot_2015-05-09_at_22.27.21.png)

Push event with multiple commits and a Compare link:

![Screen_Shot_2015-05-09_at_22.28.01](https://gitlab.com/gitlab-org/gitlab-ce/uploads/a70c718e559f69177f9341c0af92707c/Screen_Shot_2015-05-09_at_22.28.01.png)

See merge request !632

Don't symbolize params.

See merge request !636

Migration Style Guide

Fixes #2305

See merge request !1821

Make omniauth-kerberos dependency optional (add group 'kerberos')

Follow shell command guidelines in Brakeman task

There is no real issue here because the shell command is completely
static, but it is still nicer to just follow the guidelines in
doc/development/shell_commands.md .

See merge request !1819

There is no real issue here because the shell command is completely
static, but it is still nicer to just follow the guidelines in
doc/development/shell_commands.md .

Don't show duplicate deploy keys

This fixes the issue described in [#1516](https://gitlab.com/gitlab-org/gitlab-ce/issues/1516) where you would see the same deploy key multiple times if it was used by multiple projects.

See merge request !629

This makes it consistent with the Invalid email/password error message
from the previous step.