This commit mirrors the commit a18b641
by @bbelderbos which fixes this issue in days/053-056.

This commit is mirroring ab86ca9 in
days/053-056.
Previously, the registration form would not show any field errors
that might occur during registration. This commit updates the
`registration_form.html` template in the demo as well as the starter
code to show occurring field errors.

Before, it was possible in the api font-end for a logged-in user to set
the user value of a quote during create or update. This means, the user
could set another user as the owner of the quote.
This behavior seems unintended.
To prevent this option, the user field is defined as a hidden field in
the serializer. Hidden fields will not show up in the api frontend form.
A quote should have a user assigned to it (even if it is not required on
the model level) to allow management (update/delete) of the quote. The
quote can only be managed by it's owner. To prevent quotes without an
owner, the hidden user field is assigned the default value of the
currently logged-in user in the serializer.