Fix behavior of the channel media-only warnings

[luni3359]

The current warning system serves the purpose of keeping media-based text channels from getting flooded with conversations that should go in different text channels (which are not entirely disallowed, but are discouraged).

How it currently behaves:

1. Counter keeps track of how many messages have been sent in any given text channel.
   • The counter resets when a message is sent in a different text channel.
   • Sending urls resets the counter.
2. If the counter surpasses the defined maximum, a warning is issued.

Current issues:

• Sending messages across multiple servers resets the counter, making the counter more useless the more channels the bot is in due to message frequency.
• Media-based commands don't reset the counter.

[luni3359]

This feature will come with the (proper) introduction of a database. It will save the count of each text channel there, and the time when it was last sent.

"Small talk" growing into a full-blown conversation will make the bot ask to stop it. For example, people start sending messages to each other (somewhere around 25 in a short span) and it will trigger the bot. A greater limit will allow it to permit further messages as long as enough time has passed since the last message sent.

[luni3359]

Starting from commit 8355c7773732887e1bb37936049bcff45425f024 warnings now reset with any kind of command issued.