Unwanted profile creation

Fixes #7

As discussed, this PR includes:
* a Doctrine event listener to attach default profiles (which can't rely on a service using Doctrine),
* a task to migrate pre-existing users so they too have a default profile,
* the removal `checkUserHasProfile` and left joins to the profile table, and,
* tests to verify the behaviour of the task and listener.

I ended up not merging the Profile and User model just yet. There are some issues I'd like to address before that, mostly to do with interfaces.