Add warning banner when a user deletes all global preferences

If a user deletes all global preferences add a warning banner in the preferences pages alerting that he will not receive any preferences. Something like:

Warning!

All global notifications are disabled! You will not receive notifications unless you define global or channel preferences.

Read more on: https://codimd.web.cern.ch/Ho_ejjccSNOnjG5gaJAF6A#3-Possibility-to-turn-off-alltogether-preferences-for-a-channel-or-all-channels