For one reason or another, we have ended up with a few tags with commas, even though we don't support them (they were available for adding from the API for example). See https://gist.github.com/reosarevok/5423ee17190f7668ee03e2116d49f280 for the current list.

We should get rid of them since they cause issues (see MBS-8697). Looking at the list, there are two main sorts of these tags. A few are genre lists, and should ideally be replaced with all the (comma-separated) tags in the list separately. The rest are tags where mostly dropping the commas seems preferable to dropping the tag, so we should keep it but drop the commas.