Add tagging

For events, add the ability to add meta-tags. Specifically this could be music-genre, type of event and other things. This should allow users to search based on tags, but not limit to a specific type of information, but allow anything to be sorted by these labels.

No files, tests, or entry points are identified in the issue. First clarify the event tag model, supported search behavior, and how tags should be managed; done means users can add arbitrary event tags and search events by them without restricting tag categories.

Written by the indexing model from the issue text.