Make configurable who can receive impulse notes

It might be desirable that only persons who have an active profile can receive impulse notes, or only persons who are in a specific group.

This shall be implemented in the following way:

• Add a preference defining who can receive impule notes, with three levels, defaulting to "With explicit permission":
  • Everyone
  • With active profile
  • With explicit permission
• Add a preference who can see their own pedagogic overview, with the same three levels, defaulting to "Everyone"
• Add a preference who can write impulse notes about others, with the same three levels, defaulting to "With explicit permission"
• Add a permission/rule can_receive_impulse_notes, honouring the preference
• Align all other rules to honour the respective preference