Prevent accidental password change

[sualko]

Describe the bug If a user changes the access method after he shared the room password with other people, the password will change and he is not able to restore the previous password.

To Reproduce Steps to reproduce the behavior:

1. Set access method to password
2. Invite people
3. Change access method

Expected behavior User should be warned about the password change, or he should be able to recover. Another option would be to generate a new password only on user request.

Additional context Alternative we could also implement commands to occ which allows an admin to change the password.

cc @spreeni151