Add group flood control

[alvarogzp]

The bot should:

• Kick or restrict writing a user who sends too many messages per second/minute (configurable).
• Notify the group when it takes some action (eg. the user has been restricted/banned).
• The feature should be opt-in per group, being off by default.

Nice to have:

• Warn a user before reaching a limit (eg. when a user has written 80-90% of allowed messages per minute and is about of being restricted).
• Warn when enabling the feature on a group when he is not admin, as he won't be able to perform any action. It could however run in a "warning" mode, sending messages when a limit has been reached, or even mention an admin or notify on another group.
• Allow group admins to override bot actions (this could be implemented using an inline keyboard on the message that tells about the action with buttons to override the action).