Add requires_zed_cla column to channels table

Don't allow granting guests write access in a call where the channel
or one of its ancestors requires the zed CLA, until that guest has
signed the Zed CLA.

Co-authored-by: Marshall <marshall@zed.dev>

crates/collab/src/bin/seed.rs

@@ -68,7 +68,7 @@ async fn main() {
             user_count += 1;
             db.get_or_create_user_by_github_account(
                 &github_user.login,
-                github_user.id,
+                Some(github_user.id),
                 github_user.email.as_deref(),
             )
             .await