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/integration_tests.rs

@@ -5383,7 +5383,7 @@ async fn test_random_collaboration(
                 false,
                 NewUserParams {
                     github_login: username.clone(),
-                    github_user_id: (ix + 1) as i32,
+                    github_user_id: (ix + 1) as u32,
                     invite_count: 0,
                 },
             )