Save content state in browser local storage (settings)

[otacke]

Pull request h5p/h5p-php-library#81 will allow H5P to store the current content state in the browser's local storage. It needs to be turned on though.

When merged in, admins can set the current state to also be stored in the browser local storage, so student can resume their exercises even if they are not logged in.

Requires h5p/h5p-php-library#81 to take effect.

Implementation details

• Add select field to WordPress options labeled "Users who may resume tasks" with options
  • "Only users who are logged-in" (= current behavior when "save content state" is activated; default)
  • "All users" (= storing in database and local storage)
• New option will only be visible when "save content state" is activated
• Bonus: Frequency for saving option will also only be visible when "save content state" is activated

[bernhardkaindl]

@icc, can you review this or somebody else?