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

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.

Didn't bother with files in db/, config/, or features/

Also adds specs to the two affected classes that would have caught my
dumb mistake.

Any associations, validations, delegates, scopes and methods that
were exactly the same in both Issue and MergeRequest models have been
moved to a new IssueCommonality module (role) that gets included by each
class.

There was actually quite a bit of duplication, because MergeRequests are
basically just specialized Issues.