Moves issue specific code to IssueQuery.
Adds STI to Query model. Issue queries are now IssueQuery instances.
Use inheritable class attributes in Query model.
Removed conditions on HTTP methods.
Use \A and \z in validation regexps.
Cleanup dependencies in Project model.
Moved user status constants to Principal model.
Specifies HTTP methods for all routes.
Fixed that issues grouped by custom field is broken in PDF export (#12513).
