Todo: Add time limit to keyword tracking "assume presence"

[FloatingMilkshake]

The assume_presence option, if set for a keyword, will avoid notifying the tracking user of the keyword mention if the message containing the keyword was sent directly after a message sent by the tracking user.

This usually makes sense—but not always! For example, if the tracking user is having a conversation and a tracked keyword is mentioned 5 seconds after their last message, it is probably safe to assume they will see it—we can skip DMing them to avoid spam.

However, if the channel is not very active, and the last message happens to be from the tracking user, and then a message containing a tracked keyword is sent 4 hours later—they still will not receive a DM. It is much less likely that the user will see this message, so we should send a DM anyway, even if assume_presence is enabled for this keyword.

The question is—how much time is appropriate? Should this be a setting users can configure per-keyword?