collab: Allow enabling feature flags for all users (#16372)

This PR adds a new `enabled_for_all` column to the `feature_flags` table
to allow enabling a feature flag for all users.

Release Notes:

- N/A

crates/collab/migrations/20240816181658_add_enabled_for_all_to_feature_flags.sql

@@ -0,0 +1 @@
+alter table feature_flags add column enabled_for_all boolean not null default false;