Make the calendar-selection widget smaller

[nmlorg]

The shared bot has 13 global calendars, and I'm about to add a 14th, so the calendar selector is going to end up with 15 buttons (including Back). At 14, the button list already scrolls off the screen (and the actual message text is completely hidden), so it might make sense to either do two columns of buttons or, if it ends up saving space, doing a geographic hierarchy like /groups.

[nmlorg]

This might need to be split into 2 or 3 issues, but additional ideas:

Replace location with tags in /groups, and reuse the same logic for calendars. Instead of strictly being organized geographically, groups, calendars, etc. can have tags based on geography, topic, etc., which would be flattened rather than hierarchical. (Though another alternative would be to go the other direction and make things super hierarchical—instead of a flat geographic listing, as used in /groups now, you might first select country, then state/province, then city; and at the same level as country might be just a "topics" overheading, or maybe even "topics" and "geography" at the top level, etc.)

Unrelatedly, 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). It might also make sense to be able to configure a separate time zone for each group(?). Possibly other calendar-specific (rather than user-specific) settings. (Though another approach might be to create user-specific filter settings, which are applied to all calendars enabled by the user—or group admin.)