Allow the relevant people to see private notes

Originally created by @sajolida on #11359 (Redmine)

While working on accounting and fundraising issues, we started using
private tickets and private notes. Private notes where even added to
private ticket to prevent publish stuff by mistake is unmarking a whole
ticket as private. Still, from tails/fundraising#11343#change-56534 it seems like people
can’t see private notes even if the ticket is assigned to them, but they
can still see the ticket.

We should either:

• Allow more people to see private notes.
• Stop using private notes inside of private tickets.
• Stop using private tickets all the way.
• Anything other option?