Restrict the showcase bot to avoid false positives

[bvisness]

People get spammed by the "can't find a project with that tag" message when they forget to use a code block in channels like #help. At the very least the bot should not be active in threads, but perhaps we should just restrict it to a few channels again.

[bvisness]

Ok, this is getting ridiculous.

These are all on the home page right now. The last message is from a #help thread, the middle from #orca, and the first message literally just from a #network-meta discussion about this bug. This is absurd. We should probably restrict this to just #project-showcase, #jam, and #workspace, and make sure to exclude all threads everywhere.