Refactor mod creation route, add more checks (#80)

This also removes the `team_members` field of `InitialModData`, as
team members are no longer specified at mod creation.

src/database/models/ids.rs

@@ -88,7 +88,7 @@ generate_ids!(
     UserId
 );
 
-#[derive(Copy, Clone, Debug, Type)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Type)]
 #[sqlx(transparent)]
 pub struct UserId(pub i64);