Invited user unable to access /spacedeck sketches

[aofn]

Describe the bug
When a sketch is sent as an invitation to another user #42, the invited user is unable to open the sketch if it is set to private (which they are by default).
On top of that, users can't to change the visibility of sketches through /spaces.

To Reproduce
Steps to reproduce the behavior:

1. Create a new sketch
2. Send an invitation to another user to access the sketch (if Invite users to service rooms  #42 is not yet merged you can checkout the branch or use element).
3. Log in as the invited user and attempt to open the sketch.
4. The iframe shows the spacedeck landing page

Expected behavior
Invited users should be able to access and open sketches they've been invited to, regardless of the sketch's visibility setting.

Screenshots

Suggested solution
The most straightforward solution to this issue would be to set the visibility of newly created sketches to public by default.

Alternatively, we have to simultaniously invite people to a spacdeck sketch with the spacedeck api (have not checked if this is possible) and keep the two in sync. Which imo is not worth the hassle of a private sketch atm.