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 45eb4d2261ed0d943fd503a6d79ee3b7b7558c09)