UI testing as fake user

[ml-evs]

Bugs like #1361 can't be caught unless we allow our e2e tests to fake user/admin login.

This should be possible to do via the magic-link mechanism, i.e., we insert a predefined magic link in the database and let the UI tests login with it. This might be a little bit trickier to handle for local tests.

Some simple UI tests can then be written for functionality that only works when the user is logged in, e.g., the admin dashboard.

[ml-evs]

Potential next steps:

• Try to take all of our existing cypress tests, and run them as if the testing thing is logged in as a user
• If successful, try tests that also log out and log back in as a different user to make sure the correct samples are visible