Sign up flow after login of new users

[steffen]

After the user logs in for the first time, they should have the option to select their conditions.
Posts that they create are automatically tagged with that condition.
In the discovery feed they can filter between seeing all posts on Rarely, or only the ones tagged with their conditions (or logic).
We might create the concept of "condition groups", such as "neuromuscular diseases", for better filter options?