nixos/nextcloud: write config to additional config file

One of the main problems of the Nextcloud module is that it's currently
not possible to alter e.g. database configuration after the initial
setup as it's written by their imperative installer to a file.

After some research[1] it turned out that it's possible to override all values
with an additional config file. The documentation has been
slightly updated to remain up-to-date, but the warnings should
remain there as the imperative configuration is still used and may cause
unwanted side-effects.

Also simplified the postgresql test which uses `ensure{Databases,Users}` to
configure the database.

Fixes #49783

[1] https://github.com/NixOS/nixpkgs/issues/49783#issuecomment-483063922