Add a Telegram bot to send announcements and reply to questions. (#1305)

This bot is inspired by #1198 and by the ioi-germany implementation
in https://github.com/ioi-germany/cms/blob/main/cms/service/TelegramBotService.py.
It is implemented as a separate service, and does not modify the
database.

The contest admin must configure a chat ID in which the bot will send
questions as they arrive; they will be kept up to date as they are
replied to or ignored. Announcements are also sent to this channel.

Members of the channel can reply to questions, either with pre-baked
answers (with the inline keyboard) or by replying to the messages (and
specifying `/answer`); they can also send announcements.

The bot supports running for all the contests, in which case it only
interacts with questions.

Author: veluca93