Adds bp_is_{component}_directory new helper functions

To check if the current page is the activity, members, groups or blogs directory page, you can now use these new helper functions.

Props r-a-y, DJPaul

Fixes #5408

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7926 cdf35c40-ae34-48e0-9cc9-0c9da1808c22