Implement Policies

In the fingerprint PR we had a little discussion about security. I mentioned safetynet and even though it was meant as a joke, now when i think about it it's actually not that stupid. In order to give admins more control about what users are allowed and have to do, I request to introduce policies that can be set by the admin. Those policies can be things like:
- require passcode or fingerprint authentication
- disable passcode fallback if fingerprint is is configured
- require safetynet to pass in order to use the app
- disable webdav (so safetynet actually makes sense)
- require device to have a pincode/unlock pattern etc. set
- disable access for old app versions

I think that would be a great benefit, especially when looking at how nextcloud is used in enterprises. Also when the password manager will finally be introduced, even in smaller setups such policies would be useful to force users to comply to company policies or just to make the own system safer.
However this would require a lot of changes and so I am asking for comments here if that feature is wanted and if there are any other options that should be added.