Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

Adds UTC time to the "Today's Date" block, and a fuzzy/relative
timestamp to all other displayed dates.

ECOM-2807

I also took this opportunity to do some small cleanups of the date
summary code, specifically converting some attributes to properties in
order to remove a Pylint directive disabling warnings about missing
docstrings.

Enabled behind the
`SelfPacedConfiguration.enable_course_home_improvements` flag.

ECOM-2604