Update meeting-hours API to support edge cases

[ben9583]

Looking at the potential meeting times for this semester, it's looking like Web and DDR committees may meet at the same time and Finance is meeting every other week, neither of which are supported by our current meeting-hours API. Even if the former doesn't happen, it's still good to make sure we can handle this for the future.

For conflicting meeting times, we'll probably just have /api/meetings/current and /api/meetings/next return a Listpopulated by all the same date-and-time meetings instead of returning a single meeting. Should be no need to change /api/meetings/list.

As for biweekly meetings (or n-weekly meetings, for that matter), not sure how we'll keep track of off-and-on weeks, especially considering school breaks, but we'll probably also have to update schemas/meeting_hours.json and configs/meeting_hours.yaml in etc.

Anyway, I'll probably work on this myself and make a pr in both repos when that's figured out.

Relevant etc issue: ocf/etc#344