Adds TimeEntryQuery for listing time entries.
Moved build_query_from_params helper to Query#build_from_params.
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.
