Rework Tags plugin

This PR reworks the Tags plugin and introduces the Widget extensions concept.
- Widget extensions are functions that can be added to a widget as options.extensions (expects a list)
- These functions are run on the widget at rendering.
- This is a hook point to customize widgets, add necessarily new fields, etc.

The Tags plugin:
- Converted from coffee to js.
- It is no longer a plugin but an UI extension, so moved from plugins directory, to ui directory
- The filtering part is gone, since we have a separate plugin for that
- Offers two widget extensions: `configureViewer` and `configureEditor`

I don't think I like this pattern much. It saves very little typing and I can't see any other benefit. The user can simply invoke the extensions themselves with the instance of the widget.