Make it (a tiny bit) easier to run your own collab (#9557)

* Allow creating channels when seeding
* Allow configuring a custom `SEED_PATH`
* Seed the database when creating/migrating it so you don't need a
  separate step for this.

Release Notes:

- N/A

crates/collab/src/tests/test_server.rs

@@ -515,6 +515,8 @@ impl TestServer {
                 zed_client_checksum_seed: None,
                 slack_panics_webhook: None,
                 auto_join_channel_id: None,
+                migrations_path: None,
+                seed_path: None,
             },
         })
     }