Cleanup and rework.

- Changed the models to have proper references to foreignkey fields.
- Added a resource_id to the issues and a description to the categories for display
- Fixed the plugin to be more like modern ckanext plugins.
- Moved the API actions into a separate controller.
- Started removing Stream filters to make eventual migration easier.
- Moved templates into /templates/issues/
- Made sure issues works *just* using HTML, will JSify later.
- Added some fields to the get API call