Batch 1 of text improvements

Batch 1 of changes from my effort at !635 to walk through every piece of text in GitLab and see if it can be improved.

This batch includes:

- Improve text on error pages.
- Improve Git access error messages.
- Improve description of branch protection levels.
- Improve OAuth signup error message.
- Improve OAuth application flash messages.

cc @rspeicher

See merge request !642

Version check

See merge request !1509

Extract handling of relative file links to its own HTML filter

Call merge request web hook for each commits

defreeze code

Fixes #2166.

See merge request !1820

Call merge request web hook every times a new commit hits the
source branch
Signed-off-by: Arthur Gautier <baloo@gandi.net>

Replace outdated gif in README with link to website

Fixes #2298

See merge request !1806

Fix bug where avatar filenames were not actually deleted from the database during removal

This would result in a 404 error in certain views.

The `save` call was being rolled back due to an error in the validation step.
Relax the validation step so that this works.

Closes #1570

See merge request !620

Don't accidentally unblock auto created users from Active Directory.

Fixes #1581.

See merge request !638

Update docs in line with mysql-postgresql-converter changes.

See merge request !640

Update migration style guide

See merge request !1823

Show user roles by comments.

I think it would be useful to show the user's role (Owner/Master/Developer/Reporter) by comments on GitLab.com (like GitHub does with Owner/Collaborator). That way people will know that when someone from the project's core team comments, they're not just some random user, but they're actually on the team. Currently they have the gather this from context.

![Screen_Shot_2015-05-12_at_12.15.45](https://gitlab.com/gitlab-org/gitlab-ce/uploads/0da0eea2087d6d3b14a2c0c594575c8a/Screen_Shot_2015-05-12_at_12.15.45.png)

On hover, the note actions replace the role:

![Screen_Shot_2015-05-12_at_12.17.32](https://gitlab.com/gitlab-org/gitlab-ce/uploads/54a83c2f5047ea3ce0387f6692b14fe5/Screen_Shot_2015-05-12_at_12.17.32.png)

---

Addresses internal issue https://dev.gitlab.org/gitlab/gitlabhq/issues/2321.

cc @sytses

See merge request !639

Add SystemNoteService class

The Note model was basically two models crammed together - one handling user-created notes
(i.e., comments on things) and one handling system-created notes (i.e., references).
This splits out the system-specific stuff to a new SystemNoteService class.

See merge request !595

Conflicts:
	lib/tasks/brakeman.rake

Restructure project sidebar and add more icons.

I moved the repository stuff down (including the clone bar), and the project/contribution stuff up. I also added more icons, to the sidebar and tabs on other pages.

---

![Screen_Shot_2015-05-12_at_10.29.36](https://gitlab.com/gitlab-org/gitlab-ce/uploads/ada4a77a07913571e5ac4f00fdd182b9/Screen_Shot_2015-05-12_at_10.29.36.png)

See merge request !633

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