Move the 'new_member' activity type into the Members component

Since BuddyPress 1.1, the 'new_member' type was attached to the xProfile component. In version 2.2, we are moving it into the Members one. Once the user will have upgraded to this version, every existing 'new_member' activity items will have their component updated to the 'members' one and each time a new member will register, the 'new_member' activity will be attached to the members component.

Props boonebgorges, DJPaul

Fixes #5807

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9273 cdf35c40-ae34-48e0-9cc9-0c9da1808c22