Feature/organizer comment notification toggle

[zhouyijun111]

What this PR does

This PR adds an organizer-level toggle to control whether “proposal comment” notification emails are sent to organizers.

Changes

• Database: Added users_roles.email_notifications (default: enabled).
• Admin UI: Added an “Email notifications” switch on the conference organizer users list (AJAX-backed).
• Backend: Implemented Admin::RolesController#toggle_comment_notifications to persist the preference.
• Authorization: Updated AdminAbility to allow the toggle action.
• Email delivery: Updated the comment-notification recipient scope so organizer users only receive emails when email_notifications = true.
• Frontend: Improved osem-switch.js handling to prevent incorrect auto-triggering during initialization/cache restore.

Test plan

1. Go to the admin page for a conference and open the organizer user list.
2. Toggle “Email notifications” to No.
3. Create a new proposal comment.
4. Verify in letter_opener that no email is sent to that organizer.
5. Toggle “Email notifications” back to Yes.
6. Create another proposal comment.
7. Verify in letter_opener that an email is sent.

Screenshot