Fix readonly logic for text/code questions

Broken in https://github.com/inducer/relate/commit/ea36dc0dab3e2ed568f403c6af4e97fde7b0f363, part of gh-864.

The gist is that that change reinterpreted read-only as disabled, which
is not true. In django, disabled fields only provide their initial
value, not the bound value. And the DOM will not include them in form
submissions.