Enhance readability of ActionView DateHelper#distance_of_time_in_words

Refactor numerical constants to module constants which give the numbers
a contextual meaning. This commit aims to provide quicker understanding
for part of the implementation of the
DateHelper#distance_of_time_in_words method.
Merge pull request #14334 from kennym/master

Enhance readability of ActionView DateHelper#distance_of_time_in_words