nixos/redmine: Fix database assertions

Recent PR 266270[1] modified an assertion related to database settings
of the redmine service. There are two problems with that change:

1. Assert message was not updated to reflect the change in the assert
   condition.

2. The new condition applies only to postgresql, not the default
   mysql. Therefore, the assertion breaks existing mysql-based
   installations without any reason.

This commit fixes these by 1) reverting the modified assertion to the
previous value, making the message match the condition and 2) adding a
new assertion that applies only to postgresql.

[1]: NixOS#266270

nixos/modules/services/misc/redmine.nix

@@ -264,9 +264,12 @@ in
       { assertion = cfg.database.passwordFile != null || cfg.database.socket != null;
         message = "one of services.redmine.database.socket or services.redmine.database.passwordFile must be set";
       }
-      { assertion = cfg.database.createLocally -> cfg.database.user == cfg.user && cfg.database.user == cfg.database.name;
+      { assertion = cfg.database.createLocally -> cfg.database.user == cfg.user;
         message = "services.redmine.database.user must be set to ${cfg.user} if services.redmine.database.createLocally is set true";
       }
+      { assertion = pgsqlLocal -> cfg.database.user == cfg.database.name;
+        message = "services.redmine.database.user and services.redmine.database.name must be the same when using a local postgresql database";
+      }
       { assertion = cfg.database.createLocally -> cfg.database.socket != null;
         message = "services.redmine.database.socket must be set if services.redmine.database.createLocally is set to true";
       }