Add replace attendance buttons "searching for standin" and "unregister" with a single button "i cannot attend"

[Theophile-Madet]

Point 7 on https://docs.google.com/document/d/1fccRdk7pTLjx4fqbVIA9WcRs9o_ZznfB9Sa00lZQWXc/edit
See also #354

The goal is to simplify the process for members to update their attendance so that the people at the shop have an up to date overview of who is coming.

If a member clicks the "I cannot attend", the algorithm is as follow:

• If they can be unregistered, simply unregister them
• If they can't be unregistered, mark them as looking for a standin.
  For both options, the member should be notified of the consequences (for standin search, if it can't be found they get -2 points).
  Also, if they can't be unregistered, it should be clear that they still have to send a mail to the member office to explain and maybe get excused.