Allow number of tags to be configured per-category

[cellio]

We've had a couple requests, most recently https://software.codidact.com/posts/281491, to increase the number of tags a post can have. Category configuration already has a "tags" section (for required, topic, and moderator-only tags); can we add an option for the maximum number, default 5, there?

I'm asking for category rather than site-wide configuration because it requires an admin either way (we don't have a place for mods to do this themselves) and I suspect that the needs will vary by category.

https://meta.codidact.com/posts/289364