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