Resolve "Email test failure"

Instead of debugging the seeder / logic and trying to find the edge case where it fails for a certain user (would take me many hours, I'm not sure it will be worth it), I dynamically generate the expected emails using the logic from the email service. As long as we validate the logic present in the test and believe it to be correct, this test should still be useful in case we ever make more changes to the email service. If necessary, I can also look further into the seeder edge case but I expect that to take much longer.

Closes #428 (closed)