05 Authentication with JWT middleware

See the documentation of Slim JWT Auth:

• https://appelsiini.net/projects/slim-jwt-auth/

Here is the commit, that shows what things needed to be implemented for us:

• https://github.com/ICCM-EU/BOF/commit/02290f86f22929c2d14bf5b7ff7d333c473ce2e5
• Since then, some of the code has been moved to its own class, see https://github.com/ICCM-EU/BOF/blob/master/src/classes/Auth.php

Limit access to Admin only:

• We already add a payload: is_admin (see https://github.com/ICCM-EU/BOF/blob/master/src/classes/Auth.php#L37)
• This is checked in the definition of the route:
• https://github.com/ICCM-EU/BOF/blob/master/src/routes/admin.php