Speed up the "Project Network Graph" cucumber feature

This change involves stubbing out the call to `Grit::Commit.find_all`
and limiting the number of commits to 10 vs. the standard of 650 used by
`Gitlab::GraphCommit.to_graph`.

Prior to this change, this single feature required almost 3 minutes of
running time and over 2 GB of memory on my machine. Now it takes less
than 3 seconds.

Conflicts:
	app/models/user.rb

Allow filtering issues that have no milestone or assignee

Correct usage of "Can't"

Fix current class for hooks tab in admin section

Signed-off-by: Martin Bastien <martin.bastien@studiofrenetic.com>

GFM Refactoring

Makes use of 'extended' patterns to add comments to the groups, and also
reduces the total number of groups to just those that are used.

Continues #1222

Closes #1222