SA: implement rate limiting of broadcast messages

[bizzbyster]

This should be configurable by the admin.

[bhardwajaditya]

• Soft rate limiting - Will be implemeted using the current rate limiter method to prevent brute force.
• Hard rate limiting - Will be implemented using the number of messages sent in the room in the last n hours. The number of messages that can be sent in last n hours can be set by the admin.

[bhardwajaditya]

bhardwajaditya/Rocket.Chat#14