Introduces BP_Activity_Feed class, and migrates existing feeds

RSS feeds for BP activity have historically been generated on a one-by-one
basis, with separate template files used for each theme. This resulted in a
huge amount of unnecessarily duplicated code. Moreover, it made the process
of customizing feeds, or providing new feeds, unnecessarily cumbersome.

BP_Activity_Feed abstracts the central parts of RSS feed building, allowing for
easier customization, and fewer points of failure.

This changeset also fixes our RSS implementation to match best practices with
respect to encoding content inside of the <description> element.

Fixes #5020

Props r-a-y

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7207 cdf35c40-ae34-48e0-9cc9-0c9da1808c22