Always use user pk to lookup a user and/or member

Ghost Userrequested to merge
fix/member-pk-profiles into master

Closes #481 (closed)

Previous behaviour

Steps to reproduce:

  1. Go to the member page of cohort 2017.
  2. Click on Aniek den Teuling with member id 1559.
  3. Observe that you are redirected to Romy Stähli with user id 1559.

New behaviour

Steps to validate that it works:

  1. Go to the member page of cohort 2017.
  2. Click on Aniek den Teuling with member id 1559.
  3. Observe that you are redirected to Aniek den Teuling with user id 1560.

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