EE Port: Combine all datetime library functions into `datetime_utility.js`
What does this MR do?
This MR is an EE port of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22570.
This MR moves datetime methods from app/assets/javascripts/lib/utils/pretty_time.js
& app/assets/javascripts/lib/utils/datefix.js
into app/assets/javascripts/lib/utils/datetime_utility.js
.