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.sqlite/20221109000000_test_schema.sql

@@ -10,7 +10,8 @@ CREATE TABLE "users" (
     "created_at" TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
     "metrics_id" TEXT,
     "github_user_id" INTEGER,
-    "accepted_tos_at" TIMESTAMP WITHOUT TIME ZONE
+    "accepted_tos_at" TIMESTAMP WITHOUT TIME ZONE,
+    "github_user_created_at" TIMESTAMP WITHOUT TIME ZONE
 );
 CREATE UNIQUE INDEX "index_users_github_login" ON "users" ("github_login");
 CREATE UNIQUE INDEX "index_invite_code_users" ON "users" ("invite_code");