Rework views and/or schemas to support webargs 5.0

As mentioned in 664c6f67, webargs 5.0 no longer sets values for optional arguments: https://github.com/marshmallow-code/webargs/issues/342

The above-linked commit pins webargs' version to avoid this behavior change for the moment, but to be able to upgrade will require some reworking of views/schemas to be able to handle the new approach.