firefly-iii: Connect to external PostgreSQL

Since the new database server outside the Kubernetes cluster, created
for Authelia, was seeded from a backup of the in-cluster server, it
already contained the data from Firefly-III as well.  Thus, we can
switch Firefly-III to using it, too.

The documentation for Firefly-III does not mention anything about how
to configure it to use certificate-based authentication for PostgreSQL,
as is required by the new server.  Fortunately, it ultimately uses
_libpq_, so the standard `PG...` environment variables work fine.  We
just need a certificate issued by the _postgresql-ca_ ClusterIssuer and
the _DCH Root CA_ certificate mounted in the Firefly-III container.

firefly-iii/postgres-cert.yaml

@@ -0,0 +1,13 @@
+apiVersion: cert-manager.io/v1
+kind: Certificate
+metadata:
+  name: postgres-client-cert
+spec:
+  commonName: firefly
+  privateKey:
+    algorithm: ECDSA
+  secretName: postgres-client-cert
+  issuerRef:
+    name: postgresql-ca
+    kind: ClusterIssuer
+