Usability: More graceful handling of non-configured cloud services

[agwells]

If you install this artefact and the blocktypes, and then go to the new "Content -> Cloud" page without configuring an API key for each of them, you see all the blocktypes but you also see an error message for each one of them that says something about the key not being found.

You can workaround this by disabling the blocktypes you're not going to use. But it would be more friendly if the page just silently hid the cloud services that aren't properly configured.

Likewise, it'd be good if each of the blocktypes could detect that it's lacking an API key, and hide itself from the blocktype list if so.