Fix error described in https://github.com/gitlabhq/gitlabhq/issues/5185

Because notes scope was not limited by project in
MergeRequest#mr_and_commit_notes it causes comments from project A
appears in discussions for MR in project B.

Make sure the flowdock-git-hook is loaded. Seems it is not autoloader, due to the gem/lib name mismatch.

Added test for the FlowdockService.

Before we have only owner_id to determine group owner
With multiple owners per group we should get rid of owner_id in group.
So from now @group.owner will always be nil but
@group.owners return an actual array of users who can admin this group