Enable ad-hoc users to have temporary (read only) access #296

Is your feature request related to a problem? Please describe.
In a group of people making music together (a band, life group at a church, ...) we sometimes have some participants without access to songdrive. There should be a way to share access with them and give them the opportunity to see the songs and playlists in a friction free ad-hoc manner.

Describe the solution you'd like
It would be great if every logged in user could generate a shareable link and/or qr code to give other read-only access valid for a limited time period (3 hours). They should not have a real persistent user, but only a valid session with the role "reader" and a made-up name (e.g. "invited by "). The limited validity would ensure that the license constraints (e.g. the licensing framework of a church) is not violated since the access (right to copy the copyrighted songs) is only possible within the current (church) event.

Describe alternatives you've considered
Alternatives are:

• sharing of pdfs (clumsy, loosing the presentation benefits like song key adaptations and live progress)
• sharing the username and password of participants (outright bad)
• full user invitation (will create a lot of users that are not actively using songdrive, might take too much time and ceremony)

Additional context
This feature requires an "ephermal" sessions feature from amberbase, but now with our own backend it will be possible. We want to limit it to read-only, but amberbase will need to implement it more generic (since it has a more generic access rights management)