Use lang.AM and lang.PM

Fixes #331, as well as three other minor issues:

* The regex that parsed the existing time input value wasn't accounting for uppercase “AM” or “PM”
* The regex that parsed the existing time input value wasn’t escaping periods in “A.M.” or “P.M.” language strings, which are commonly used, so if the language string was “A.M.” the regex would have matched “AXPX”, etc.
* The code determining if the existing time input value was >= 12:00 PM wasn’t accounting for uppercase “AM” or “PM”.