fix emergency access invites (#4337)

* fix emergency access invites with no mail

when mail is disabled instead of accepting emergency access for all
invited users automatically, we only accept if the user already exists

on registration of a new account any open emergency access invitations
will be accepted, if mail is disabled

also prevent invited emergency access contacts to register if emergency
access is disabled (this is only relevant for when mail is enabled, if
mail is disabled they should have an Invitation entry)

* delete emergency access invitations

if an invited user is deleted in the /admin panel their emergency
access invitation will remain in the database which causes
the to_json_grantee_details fn to panic

* improve missing emergency access grantees

instead of returning an empty emergency access contact the entry should
not be added to the list. also the error handling can be improved a bit.