Use _nx() to make 'x new' activity mentions gloss translatable

Languages other than English require distinguishing between the singular and
plural adjectival forms of "new".

Fixes #4916

Props johnjamesjacoby, merty

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7004 cdf35c40-ae34-48e0-9cc9-0c9da1808c22