warn admins of slow delivery if they send attachment
context
- signal rate limits images so we do to.
- that means messages with images can take very long to send to large channels, but admins don't necessarily know this.
- as such they are both sad and likely to unwittingly clog up their channel
- so: let's warn them about what will happen when they send an image
behavior
(stub)
WHEN an admin sends a broadcast message with an attachment
- THEN they will receive a message that says something like
WARNING: your last message contained an attachment. It will take <num users * 2 seconds> to transmit to all users and your channel will not be able to support outgoing messages until it is done transmitting