Add option to mirror an upstream repository.
Closes internal https://dev.gitlab.org/gitlab/gitlab-ee/issues/279
Depends on gitlab-org/gitlab-shell!29
To do:
- [x] Decide on what user should be the author of the activity feed events. The initial project creator perhaps? That makes sense for personal projects, but less so for group projects, where it's even possible that the creator has since left the team.
- [x] Write specs!
- [x] Write documentation
- [x] Port back relevant commits to CE: gitlab-org/gitlab-ce!1822
----
## Mirror status on project homepage

## Settings navigation item

## Mirroring settings

## Activity feed with new "pushes" (mirror updates)

I have yet to decide on what user should be the author of these events. The initial project creator perhaps? That makes sense for personal projects, but less so for group projects, where it's even possible that the creator has since left the team.
## Warning that branch has diverged from upstream

## Commits heading after failed update

Link send user to the mirroring settings if they are a project admin.
## Mirroring settings after failed update

## New project page

## Import form after failed import
(Screenshot outdated)

Shown when the initial import of a project fails, not when an update of an existing project fails.
cc @dzaporozhets @sytses @JobV
See merge request !51
Gracefully handle SMTP user input errors (e.g. incorrect email addresses) to prevent Sidekiq retries
### What does this MR do?
This MR gracefully handles SMTP input errors (e.g. incorrect or invalid e-mail addresses) to prevent these types of exceptions from causing Sidekiq to retry the task. If these specific exceptions occur, they will be logged, and the e-mail will be dropped from the queue.
### Why was this MR needed?
If you include an author that has a misspelled e-mail address, Sidekiq will keep sending e-mail to all the recipients even if they have already received the e-mail. The only way to recover is to clear the Sidekiq queue.
Note that other exceptions can still be thrown (e.g. `IOError`, `Net::SMTPAuthenticationError`, `Net::SMTPServerBusy`, `Net::SMTPUnknownError`, and `TimeoutError`). If the worker encounters these, Sidekiq should retry the task.
### What are the relevant issue numbers?
Closes https://github.com/gitlabhq/gitlabhq/issues/9560
See merge request !1163
and request them each session. Pass these tokens to the project import data.
This prevents the need to encrypt these tokens and clear them in case they
expire or get revoked.
For example, if you deleted and re-created OAuth2 keys for Bitbucket, you would get
an Error 500 with no way to recover:
```
Started GET "/import/bitbucket/status" for x.x.x.x at 2015-08-07 05:24:10 +0000
Processing by Import::BitbucketController#status as HTML
Completed 500 Internal Server Error in 607ms (ActiveRecord: 2.3ms)
NameError (uninitialized constant Import::BitbucketController::Unauthorized):
app/controllers/import/bitbucket_controller.rb:77:in `rescue in go_to_bitbucket_for_permissions'
app/controllers/import/bitbucket_controller.rb:74:in `go_to_bitbucket_for_permissions'
app/controllers/import/bitbucket_controller.rb:86:in `bitbucket_unauthorized'
```
Closes#1871