Add a layer of filter between flag and block

[emmiegit]

We should add a filter enforcement level called "warn", which isn't invisible to normal users like flagging, but doesn't delete the message. Instead it provides a premade message about the filter (the reason) in channel providing context.

However to avoid confused deputy spam, we will need to track channels and last warning sent per filter so it is not sent more than once per X (configurable) minutes.

Additionally, we can increment on subsequent occurrences, and once a higher threshold is passed, the filter begins enforcing with deletions, since at that point it's spam rather than a potentially good faith message that was flagged.