[fix] Make postgres connections magically work in common setups (#352)

* Don't use the system 'postgres' database by default

* Use postgres adapter defaults

The pgx code actually goes to great lengths to make postgres connections
Just Work(tm) out of the box, including supporting `~/.pg_service.conf`,
SSL certificates, UNIX sockets if it can find a socket at a common path,
and falling back to TCP to localhost if not.

(On Windows, it won't try to use UNIX sockets, but will read credentials
from %appdata% as is standard over there.)

By applying our flags as overrides only when they're specified, database
connections should Just Work(tm) anywhere `psql gotosocial` does.

internal/config/defaults.go

@@ -34,11 +34,11 @@ var Defaults = Values{
 	TrustedProxies:  []string{"127.0.0.1/32"}, // localhost
 
 	DbType:      "postgres",
-	DbAddress:   "localhost",
+	DbAddress:   "",
 	DbPort:      5432,
-	DbUser:      "postgres",
-	DbPassword:  "postgres",
-	DbDatabase:  "postgres",
+	DbUser:      "",
+	DbPassword:  "",
+	DbDatabase:  "gotosocial",
 	DbTLSMode:   "disable",
 	DbTLSCACert: "",