Project: create forms are still usable by keyboard when disabled

This was a leftover from the project creation view work I did.

• #274

While the parent elements are disabled on the automatic or manual creation forms, you can still hit tab to move through and use the underlying elements.

This is likely more like a bug in FUI, the disabled state cascades to child elements visually, but the fields aren't actually disabled to the browser.

To fix this, we probably need to also use the disabled states on the nested form or maybe even fields. The disabled state is currently set here:

https://github.com/readthedocs/ext-theme/blob/8b1f5b9be1ed58e7e88298616ca94b4917005c1b/readthedocsext/theme/templates/projects/import_form.html#L33

Here are a few places where a disabled form might be helpful:

• Addons configuration, disabled sections/fields when Addons is disabled

Start in readthedocsext/theme/templates/projects/import_form.html around line 33, where the disabled state is set on the project creation form. Reproduce the automatic and manual creation views with keyboard navigation, then inspect how FUI propagates disabled state to nested fields. Done means disabled form controls cannot be reached or used with the keyboard; consider the Addons configuration case noted in the issue.