Feature request: Add notification capability

[nielu]

Bot should create separate role per every mission, and assign it to every user on mission signup.
Then, it will be possible to notify users by mentioning new rank on Discord

1. when mission is about to start (or hour before?)
2. when there has been some change in the mission
3. during the mission, if some technical error happens and server falls of the rails for example
4. after the mission, if some noteworthy discussion is happening

Mentions 1 and 2 should be done by bot itself, 3 and 4 by power users (?).
Role should be removed after some time passes (24h?)