Use ‘localhost’ as default name for database server

Using localhost instead of some random string is much more likely to
actually work out of the box. Also it's a lot easier for a sysamdmin to
'scan' for things that need changing.

config.toml.sample

@@ -11,7 +11,7 @@ admin_password = "listmonk"
 
 # Database.
 [db]
-host = "db"
+host = "localhost"
 port = 5432
 user = "listmonk"
 password = "listmonk"