Allow admins to edit event series/instance banners

[nmlorg]

Breaking this out from #74:

I've recently noticed more flier-style images being used in manual event announcements for groups that already use metabot's automatic announcements. They've all been in portrait rather than landscape, and so would eat a ton more screen real estate than Google Calendar flair, but that's ultimately a style choice for the group.

There are two use cases:

1. A group decides the flair automatically chosen for an event is wrong and wants to override it. This would suggest some way to tag event series rather than individual instances, either by exact title match, arbitrary pattern, or even just manually editing the flair table (just for their group/bot/calendar).
2. A group creates a flier for a specific instance (including dates, special guests, or other non-repeating details). This would require either allowing specific instances to have specific override images, or rely on the group to manually assign and then unassign/replace a series override (i.e. they might override all events titled "Club Social" with a flier giving the dates for the next social, then after it has passed would have to remember to either remove the override or change it to the next flier).

I'm currently thinking of something like:  When a bot admin scrolls through /events in a private chat, a new button labeled Edit or Admin will appear. This will open up some limited editing UI where they could ask for the auto-assigned flair, potentially change it (for exact title matches), and/or assign an override for that specific instance.

There's a question about how flair is chosen based strictly on the first event in an announcement (if it has flair, it is used for the whole announcement; if it doesn't, the announcement is sent as pure text, even if a later event in the list might have flair). It might be the case that a group creates a flier for an event on a Saturday that would be masked by an event the previous evening, so we should consider having some kind of prioritization. (For every announcement, scan all events. If any one has an instance-specific flier, use it for the whole announcement. If any one had a series-specific flier, use it for the whole announcement. The first one that has flair, use it. For groups that do things like show events a month or more out, though, anything along these lines might be really misleading, so it might need to be that we actually just scan for events within X time period of the first event in an announcement (i.e. if an event on a Saturday has a flier, use it instead of the Friday event's flair; but an event the next Wednesday wouldn't have its flier take precedence until Sunday).)

It might also make sense to add an entirely new admin UI allowing bot/group admins to preview an upcoming announcement, then manually edit the banner. This really makes the most sense as far as precision/conveying intent, but is probably not something most admins would do. (For one, nobody has actually asked me to edit flair, allow them to override a series or instance's banner, etc.  To me that suggests that, if I don't expose this super intuitively—even if doing so tends to reduce some of the UI's power—it will never be used.)