Replace i32 with u32 for database columns

We never expect to return signed integers and so we shouldn't use
a signed type. I think this was a limitation of sqlx.

crates/collab/src/db/tests.rs

@@ -430,7 +430,7 @@ async fn test_fuzzy_search_users() {
             false,
             NewUserParams {
                 github_login: github_login.into(),
-                github_user_id: i as i32,
+                github_user_id: i as u32,
                 invite_count: 0,
             },
         )