For the record, I decided in the end not to make this change.  These indices are used at a high level, i.e. with the gsp.  I believe having `CARD_INDEX_...` here would be a leak of the implementation details.

@tojofo and I disagree on this, but I believe that at a high level, what we still have is a "tab" component, despite 1) how it is implemented and 2) the fact that the "tabs" are styled differently to other tabs within the app.

I think therefore `TAB_INDEX_` is still the most descriptive name (so far).