Moved tag listing to API
Organized common paging code into abstraction
Fixed filtering logs
Moved tag autocompleting to API
Removed obsolete method call

(Removed code is already executed in tag editing jobs.)
Moved listing tag relations to API
Made tag retrieval use entity conversion again

Previously engine used raw database rows for performance boost. The
benefits were negligibly small, therefore it was changed so that it
returns full entities again. That way serializing job return values
for HTTP API should be easier in the future.
Moved tag renaming to API
Moved tag merging to API
Moved user flagging to API
Moved user (un)banning to API
Reduced boilerplate by using default privileges
Moved user listing to API
Moved user registration accepting to API
Moved user account removal to API
Simplified views in UserController