Capture images / attachments

[plexus]

When people share images on Slack we don't render that.

Compare:

This may become a bit involved since we will have to actually download and store the images somewhere on our side. Similar to the limit on message history, Slack has a limit on how many of these attachments it retains, so if we link to Slack they will soon break.

So capturing them and storing them somewhere (S3?) is really the first step, to stop us from losing this history. Rendering can come after.