Timed Announcements

[magula]

There should be a feature that lets admins store announcements to be made at a specified time, e.g. 30 mins before the end of the contest.

The main use-case for this is to give warnings at certain points before the end of the competition, which is a requirement in some olympiads' rules. Though these warnings are primarily to be made verbally, this feature wouldn't hurt for online participants (or online olympiads). See e.g. the IOI's rules:

Three warnings will be given at 15 minutes, 5 minutes, and 1 minute before the end of the competition. Each warning will be given by an audible signal. The end of the competition will be announced both verbally and by an audible signal. [...] An additional announcement will be made instructing them to leave their tables and exit the competition room.

Perhaps the easiest way for us to implement this would be in the TelegramBot. This would include an additional /announceat command that asks at what time the announcement should be sent. It should ideally be possible to give the time relative to the contest end, which would have to adapt if the contest end time changes.

No accommodations have to be made for users given extra time. These have to be informed when they have been given extra time, and should understand the general announcement does not account for that.