Move activity-answer plugins to a separate module

The way it's done now (activity types as plugins with separate config installed as optional) allows only admins to see what's available. Also this way we can have multiple activity types defined in a single module. I'd just close this but if it'll actually not allow everything to be completely clean, we can move core LMS activity types to a single sub module (lms_activity_types for example). The downside is that then it'll be a bit more complicated to site builders because they'll have to install one more module to see anything usable.

Let's leave this open to discussion.

Automatically closed - issue fixed for 2 weeks with no activity.