Reduce polling time on read only public shares

[juliushaertl]

Polling intervals that trigger the sync requests are quite frequent. We should reduce the polling internal in cases where immediate updates may not be that relevant:

• When a user is the only one in a document
• For read only sessions (maybe even higher intervals when no people with editing rights are in the document)

Relevant code parts: https://github.com/nextcloud/text/blob/ea17f697135e1e274673b4c2523004a8d7e3def2/src/services/PollingBackend.js#L31-L53

On the way doing this the logic could also be refactored a bit to be easier to understand

Assigning performance tag and enhancement since this would help reduce the server load.

[luka-nextcloud]

@juliushaertl Please add some description.