Update message preferences of a user as admin to use consistent toggle icons

In MDL-57831, notification preferences have been changed to use the toggle-on/off fontawesome icon (instead of the On/Off).

This change should be also done in "Message preferences" of a user as admin:

1. Login as admin.
2. Visit the profile for any user (for instance, s1).
3. In the "Administration" block, click the "Preferences" page.

Expected behaviour (for consistency):
On/Off icons are replaced by fa-toggle.

Current behaviour:
On/Off icons (replaced in MDL-57831 for the Notification preferences) are still there.