Support the 'answered' verb

Note I haven't checked whether the scoring format is the same for answered as passed/failed yet - this just unblocks storing the xAPI statements at all.

Merged.

I found a workaround in lms_h5p. Reverting for now, but we might want to make this controlled by a hook or similar? Either a hook that returns an array of verbs, or allow modules to alter/validate statements just before they get written?

Yes, an alter hook invoked from __construct() with cached output should be fine. We'll need to think where to cache it though (or maybe don't cache it?)

Re-titling and moving back to active.