Modify jquery.timeago to better support i18n

Use string interpolation instead of concatenation of prefix, content,
and suffix. This is motivated by the fact that Transifex does not accept
empty strings for translation (even when pgettext is used to provide
context).
Merge pull request #2267 from edx/gprice/timeago-better-i18n

Modify jquery.timeago to better support i18n