update moment.js to 2.8.4
add utils.time

consolidate some time-related utils, used in save widgets
fix regular checkpoint updates in notebook

in master, this was updating every 10 hours,
regardless of the value, due to a variety of bugs,
including using relativeTimeThreshold getters
that do not exist in moment.js 2.7.
Merge pull request #7186 from minrk/moment

moment.js-related fixes