Allow users to filter events

[nmlorg]

(Splitting this off from #47:)  Instead of just toggling calendars on or off, it might be nice to be able to do things like set event filters (like if a group includes both business meetings and social events on a single calendar, users might want to be able to only show/filter out some groups of events).

I was originally thinking of doing simple substring matching, but this will overlap with event tags to some degree, so either the filter UI will need to be flexible enough to allow both substring matching and tag matching, or the tagging system will need to allow user-created (or at least bot admin-created) tags. (That is, either this UI will need to allow people to filter out events with the 21+ tag, regardless of summary/description, as well as events that match the pattern /Committee Meeting/, etc.; or the tagging system will need to allow users (or bot admins) to create tags (like business) based on pattern matching, even if an event has an explicit Tags: field.)