Adapt core to allow BP Attachments to be into the list of optional components

So that next "feature as a plugin" Attachments can be activated just like the other BuddyPress components, we need to create two new hooks.

props johnjamesjacoby, boonebgorges, imath.

See #5429

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8160 cdf35c40-ae34-48e0-9cc9-0c9da1808c22