UX: Update styling of readonly values in signup form (PR #13886)

During some authentication flows (e.g. external auth with validated emails), some fields on the signup form are readonly. Previously, they were rendered in a simple <span>, with no associated label. This commit makes them render in a disabled <input> field, so that the styling matches the rest of the form.

A subtle background is added to the disabled input to distinguish them from editable inputs.

Before:

After:

GitHub

Great idea + changes David! :+1: