Sync all repos

Scripts and documentation for moving repos, used on gitlab.com.

See merge request !1439

Added 3  rake tasks for repository maintainance

## What does this MR do?
This MR adds 3 rake tasks

- gitlab:git:repack
  - `-a`
  - `--quiet`
- gitlab:git:gc
  - `--auto`
  - `--quiet`
- gitlab:git:prune
  - Needs git version > 1.8.4.1, Ubuntu repos @ 1.9.x

## Are there points in the code the reviewer needs to double check?
AFAIK this MR abides by the Guidelines for shell commands. Also, the output given is not the diskspace saved etc, just if the commands were succesfull. (Parsing output etc did not seem like the way to go)

Output might be verbose when a lot of repo's are in the system?

## Why was this MR needed?
`git gc` and `git prune` can reduce storage space used.

## What are the relevant issue numbers / Feature requests?
Closes #1529

## Screenshots (if relevant)
`rake -T`
![Screenshot_from_2015-09-22_14-57-59](https://gitlab.com/zj/gitlab-ce/uploads/4abfa00ce7afcc73f553d92581246731/Screenshot_from_2015-09-22_14-57-59.png)
(git fsck now removed as IMHO it doesn't add any value.)

See merge request !1388

Fix signin with OmniAuth providers

OmniAuth CSRF protection was broken with the move to Rails 4.2 since
the CSRF logic in Rails changed. 

This new implementation calls out to Rails instead of copying its code,
which is far easier to maintain.

See merge request !2019

Bump gollum-lib to 4.1.0 and fix dependency mismatch with rouge

Closes #3767

See merge request !2017

Serve LFS object

Depends on gitlab-org/gitlab_git!57

See merge request !1976

Closes #3767

Add more custom validators

These custom validators allow us to DRY up our models a bit.

See merge request !1944

Fix 500 error when creating a merge request that removes a submodule

Fixes #3476 

See merge request !1989

Add explicit reference to rouge 1.10.1

We are relying on rouge 1.10.1 but nothing is referencing to this version and it is easy to lose this dependence. Previously, it was updated directly in Gemfile.lock which is not reliable way :)

https://gitlab.com/gitlab-org/gitlab-ce/commit/e20529cec4f6de0c2c1589da99abb8714ea1c1c1

`gollum-lib` has a dependence `rouge 1.9` which is old for us.

See merge request !1937