Calculated value not to be tabbable in view mode

[avernet]

We consider calculated values added by form authors in the form as "fields", and would like them to behave not unlike input fields from a usability perspective, in particular with regard to tabbing and screen reading. So, for consistency with input fields, calculated values should be tabbable in edit mode, but not tabbable in view mode.

(From a practical perspective, this improves the usability of the view page for instance when generating licenses, so one not using the mouse doesn't have to tab through lots of fields to reach the buttons at the bottom of the page.)