Make TelegramBot Persistent

[magula]

The TelegramBot is not persistent currently. This means that when it starts, it knows nothing about the messages it has sent before. (Some) consequences of this are:

• If there have been new questions it hasn't notified about (because they have been asked when TelegramBot was stopped), it will not know that, and no one may notice.
• To answer a question, you press a button under the corresponding question message, or respond to it. If TelegramBot has stopped in the meantime, it will have forgotten about its message, and so it will not register your answer.

See also cms-dev#1198, which includes suggestions on how to implement persistence.