Merge branch 'permission-improvements' into 'master'

Update permissions for issue tracker management

Don't allow guest or reporter to set assignee, milestone and label when create or update new issue and merge request.
After this change `Guest` and `Reporter` rule is used to report issues but only `Developer` and higher roles can manage issues (schedule milestone, assign to user or close any issue)

Also I removed some duplication code between issues and merge requests and put all issuable partials in one directory

See merge request !890