collab: Restrict usage of the LLM service to accounts older than 30 days (#16133)

This PR restricts usage of the LLM service to accounts older than 30
days.

We now store the GitHub user's `created_at` timestamp to check the
GitHub account age. If this is not set—which it won't be for existing
users—then we use the `created_at` timestamp in the Zed database.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>

crates/collab/migrations/20240812204045_add_github_user_created_at_to_users.sql

@@ -0,0 +1 @@
+ALTER TABLE "users" ADD COLUMN "github_user_created_at" TIMESTAMP WITHOUT TIME ZONE;