Use strict date format checking during imports

Prevent false positives by enforcing YYYY-MM-DD (used by most web journals)

The last true enables a very strict parsing mode.

DayJot inputs each line into moment() to see if the line marks a new entry. False
positives were causing absurd entry breaks into years like "1981".

moment() is very liberal and browser-dependent without strictness enabled. For
example, moment("This year, I took CS 101.").isValid() returns true in Chromium 39.