Enable adding overrides for rollout feature flags

Created by: thenamankumar

closes: https://github.com/sourcegraph/sourcegraph/issues/35795

This PR enabled adding org/user overrides for rollout feature flags as well which was earlier restricted to boolean feature flags.

This also performs minor refactoring over the GetUserFlags method in the database store. There are nested loops present which need not be.

Test plan

1. Visit /site-admin/feature-flags
2. Create a new rollout feature flag
3. Add overrides for user id and org id
4. Use console to verify working of overrides.
5. Test deleting overrides