Save content state in browser local storage

[otacke]

When users are logged in, H5P allows to store the current content state in the database. This allows students to resume their exercises where they return later. I have been asked multiple times if there was a solution that does not require users to be logged in.

When merged in, this code allows students to resume their exercises when they return without the need to be logged in. It has already been confirmed by the H5P core team that this would be a useful feature, cmp. https://h5p.org/comment/29153#comment-29153

Implementation details

• optionally store state in browser local storage item when H5P is set to (via H5PIntegration)
• retrieve previous state from local storage item if given, but previous state in database will take precedence (when students are logged in)
• delete local storage item when content type is changed (same as reset for database)
• multi-site installations are supported by having a site specific identifier in the local storage item