Added a new user group and some improvements to mr email subjects

list of changes:
- merged with 6-5-stable upstream version
- merged with a branch from another pull request https://github.com/mendyp/gitlabhq/pull/8
- added another group called "developers with merge" which have all the same rights as developers, but can also "accept and merge"
- disabled "delete old branch" checkbox when merging from protected branch 
- allowed filters on merge request page to work together, not just one at a time 
- fixed css for accept/reject/etc buttons on merge request detail page
- added "closed" and "merged" states to mr state filter
- mr email subjects became more informative

jira task - https://prodegellc.atlassian.net/browse/SWAG-522