[Tag] initial implementation of Tag model.
[TagList][Event] TagList handles converting between strings listing tag titles and the tags on an item (such as an Event).
Adding and removing tags for an event via the form view.
[EventsController][Event] show tags for events.

New `Event.tagged` scope that takes a single tag string to constrain searches to events that have the given tag.

Added a `tag=` parameter on the `events_controller#index` action to constrain the found events by the given tag.

`events_controller#show` now includes a list of tags, linked to the corresponding `index?tag=` actions.