HFP-3615 Add support for the H5P OER Hub

[otacke]

When merged in, will add support for the H5P OER Hub:

• Registration on the settings page (depending on capability)
• Getting shared content from the OER Hub (depending on capability)
• Sharing, syncing and unsharing content on the OER Hub (depending on capability)

It may be a little rough around the edges, because I do not have access to the server, obviously. I could not test everything and don't know what exactly the flow of shared/synced states is.

[bernhardkaindl]

@icc, can you review this or somebody else?