Replace mentions of 'blog' with 'site' throughout

This brings us into line with WP's terminology.

Fixes #5006

Props trishasalas

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7352 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Pinking shears

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7353 cdf35c40-ae34-48e0-9cc9-0c9da1808c22