time-util: don't use plural units indiscriminately

format_timestamp_relative currently returns the plural form of
years and months no matter the quantity, and in many cases (for
durations > 1 week) this is the same with days.

This patch changes this so that the function takes the quantity into account,
returning "1 month 1 week ago" instead of "1 months 1 weeks ago".

(cherry picked from commit 45eb4d22)