Improve UX for managing inspection times

[LukeTowers]

Porting this from #41 (comment).

The current interface for managing aircraft's TTNI, TTSN, & TNI is apparently confusing to some users, leading to incorrect values being entered and used.

We could probably help prevent that error from happening in the future by improving the language used on those fields to make it more explicit what they are for and how to use them (although there's still the problem of TTNI not being updated after an inspection all of the time).