…sql15

Closes NixOS#216989

First of all, a bit of context: in PostgreSQL, newly created users don't
have the CREATE privilege on the public schema of a database even with
`ALL PRIVILEGES` granted via `ensurePermissions` which is how most of
the DB users are currently set up "declaratively"[1]. This means e.g. a
freshly deployed Nextcloud service will break early because Nextcloud
itself cannot CREATE any tables in the public schema anymore.

The other issue here is that `ensurePermissions` is a mere hack. It's
effectively a mixture of SQL code (e.g. `DATABASE foo` is relying on how
a value is substituted in a query. You'd have to parse a subset of SQL
to actually know which object are permissions granted to for a user).

After analyzing the existing modules I realized that in every case with
a single exception[2] the UNIX system user is equal to the db user is
equal to the db name and I don't see a compelling reason why people
would change that in 99% of the cases. In fact, some modules would even
break if you'd change that because the declarations of the system user &
the db user are mixed up[3].

So I decided to go with something new which restricts the ways to use
`ensure*` options rather than expanding those[4]. Effectively this means
that

* The DB user _must_ be equal to the DB name.
* Permissions are granted via `ensureDBOwnerhip` for an attribute-set in
  `ensureUsers`. That way, the user is actually the owner and can
  perform `CREATE`.
* For such a postgres user, a database must be declared in
  `ensureDatabases`.

For anything else, a custom state management should be implemented. This
can either be `initialScript`, doing it manual, outside of the module or
by implementing proper state management for postgresql[5], but the
current state of `ensure*` isn't even declarative, but a convergent tool
which is what Nix actually claims to _not_ do.

Regarding existing setups: there are effectively two options:

* Leave everything as-is (assuming that system user == db user == db
  name): then the DB user will automatically become the DB owner and
  everything else stays the same.

* Drop the `createDatabase = true;` declarations: nothing will change
  because a removal of `ensure*` statements is ignored, so it doesn't
  matter at all whether this option is kept after the first deploy (and
  later on you'd usually restore from backups anyways).

  The DB user isn't the owner of the DB then, but for an existing setup
  this is irrelevant because CREATE on the public schema isn't revoked
  from existing users (only not granted for new users).

[1] not really declarative though because removals of these statements
    are simply ignored for instance: NixOS#206467
[2] `services.invidious`: I removed the `ensure*` part temporarily
    because it IMHO falls into the category "manage the state on your
    own" (see the commit message). See also
    NixOS#265857
[3] e.g. roundcube had `"DATABASE ${cfg.database.username}" = "ALL PRIVILEGES";`
[4] As opposed to other changes that are considered a potential fix, but
    also add more things like collation for DBs or passwords that are
    _never_ touched again when changing those.
[5] As suggested in e.g. NixOS#206467

As it is technically a breaking change, we should at least make a strong deprecation
of `ensurePermissions` and leave it in the broken state it is, for out of tree users.

We give them a 6 months notice to migrate away by doing so, which is honest.
In the meantime, we forbid usage of `ensurePermissions` inside of nixpkgs.

…DB name

It is hard to figure out which one is offending without the database name.

pgbouncer test is special in the sense where it actually tries
to connect via SCRAM SHA, let's avoid `ensureDBOwnership` here
otherwise for some reason pgbouncer will try to look in pg_shadow
for the authuser…

Given that SourceHut uses unfortunate defaults for database name, it will not
be realistic to fix this in time for 23.11.

We will leave the workaround and leave it to SourceHut maintainers to pick up the work
to clean this up after 23.11.

Mobilizon can have a custom database username and it is not trivial to sort out how to remove this.

In the meantime, for the upcoming 23.11 release, I apply the classical workaround
and defer to Mobilizon's maintainers.

Invidious uses a strange setup where the database name is different from the system username
for non-explicit reasons.

Because of that, it makes it hard to migrate it to use `ensureDBOwnership`, we leave it to Invidious' maintainers
to pick up the pieces.

…tion