Add blocklist/passlist for xAPI verbs

Think about this ...

## Pros
* Reduce storage requirements
* (Better overview - not really since we have filtering per column)

## Cons
* Suggests using xAPI as a "better" results interface only

It might be better to implement https://github.com/h5p/h5p-php-report into the H5P WordPress plugin or to create a separate one for this job.