Allow multiple activity types per answer plugin, add a null plugin

It's very important actually, it installs all activity type bundle config, binding it to the plugin it uses.
Yes, we need documentation on creating activity-answer plugins with that config for module developers, best to add a guide here on d.o. documentation pages while LMS core will simply provide working examples (it already has 2: long answer question and a true / false question).

Actually.. I have an idea how to improve that.. Updated the issue.

One question:

Provide a NULL plugin in addition to the existing ones for read-only activity types.

At the moment even slides which have zero interaction record an 'answer' in that they can be automatically scored and the individual submission is logged - does a null plugin mean a generic read-only answer or that we actually wouldn't store anything at all?

I think we can skip storing answers for such activities, what's more, we could allow question-answer plugins to prevent saving answers and handle that on their own. I'd do that as a follow-up though and store answers with empty data for now not to make this issue too big.

No reviews -> merged although I was hoping to get some input on this. We'll have follow-ups if needed.

Sorry reviewing after the fact.

A single plugin ID that can be used with multiple types is a good idea, then when there's only form config, no/less boilerplate.

It's not clear to me why the _activity_type bundle needs the leading/trailing underscores, why not just 'activity_type' there?

Thanks, updated according to what we agreed on Slack.

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