Auto-archive feature counts join/leave messages as channel activity

[agarzola]

Channels such as #investing meet the no-activity-for-30-days criteria for auto-archival eligibility, yet did not get the message from devanoobot. I think other activity, such as members joining/leaving counts as messages in the channel, something the logic does not take into account. We should check for that kind of message and strips them out so they’re not counted.

AC:

• Devanoobot correctly identifies join/leave messages and removes them from the pool of messages before determining the channel’s eligibility for archival