Support CERN phone device

Backend will now create by default a CERN phone device (https://gitlab.cern.ch/push-notifications/backend/-/merge_requests/225).

Webportal must make a few adjustments:

• Not allow deletion
• Link to help section explain what this device is and how to set one if the user doesn't have one
• Show something if the phone isn't set
• Change icon for mattermost, use phone icon for this new device