Step 1:
* Add milestone_id in the model for merge_requests
* Make it possible to create or update a Merge Request with an attached
  Milestone detail
* Add the possibility to filter by Milestone and / or Assignee in the
  Merge Requests listing page

default_branch now defaults to nil, not 'master'. It will be set after the first
push by discover_default_branch.

Merge branch 'master' into non-interactive-aws-install

Conflicts:
	doc/installation.md

Fix merge mess in installation.md

This change was made in a migration in commit 17a88bb6.
Because the schema.rb is checked in, the column size increase should
be here, too.