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/src/seed.rs

@@ -1,6 +1,7 @@
 use crate::db::{self, ChannelRole, NewUserParams};
 
 use anyhow::Context;
+use chrono::{DateTime, Utc};
 use db::Database;
 use serde::{de::DeserializeOwned, Deserialize};
 use std::{fmt::Write, fs, path::Path};
@@ -12,6 +13,7 @@ struct GitHubUser {
     id: i32,
     login: String,
     email: Option<String>,
+    created_at: DateTime<Utc>,
 }
 
 #[derive(Deserialize)]
@@ -107,6 +109,7 @@ pub async fn seed(config: &Config, db: &Database, force: bool) -> anyhow::Result
                     &github_user.login,
                     Some(github_user.id),
                     github_user.email.as_deref(),
+                    Some(github_user.created_at),
                     None,
                 )
                 .await