Support configurable attachment size in Application Settings page

### What does this MR do?

This MR provides the ability to configure the maximum size of an attachment inside a note. A parameter has been added to the Application Settings page.

### Are there points in the code the reviewer needs to double check?

What should be done with the legacy note attachment validation? I added code to make the validation work with the configurable setting. I could see an issue where an admin lowers the limit from 10 megabytes to 5 megabytes, which could cause an existing model to be invalid.

### Why was this MR needed?

We often have attachments that exceed 10 MB, and it would be nice to be able to override the defaults.

### What are the relevant issue numbers / [Feature requests](http://feedback.gitlab.com/)?

See Issue #1258

### Screenshots

Before:

![Screen_Shot_2015-03-29_at_3.06.53_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/6013a1dbc8cf61a63e93744149937fa0/Screen_Shot_2015-03-29_at_3.06.53_PM.png)

After:

![Screen_Shot_2015-03-29_at_3.12.34_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/f3518af7e8653ba40f0a3579456da6ad/Screen_Shot_2015-03-29_at_3.12.34_PM.png)

See merge request !407

Archive repositories in background worker.

Depends on https://gitlab.com/gitlab-org/gitlab_git/merge_requests/17 being merged, a new `gitlab_git` being released and this MR's `Gemfile.lock` being updated..

See private issue https://dev.gitlab.org/gitlab/gitlabhq/issues/2173.

To do after this is merged: Update https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/files/gitlab-cookbooks/gitlab/templates/default/sv-sidekiq-run.erb in omnibus.

See merge request !436

Fix bug where error messages from Dropzone would not be displayed on the issues page

Closes #1258

Respond with full GitAccess error if user has project read access.

Should help with debugging #1236.

cc @marin

See merge request !437