Follow Blogs: Add "Activity > Followed Blogs" profile nav item.

See #31.
Fix "Sites > Sites I Follow" filtering when there are no matches.

Previously, passing the value of '-1' when there are no matches would result
in a match of blog ID 1 due to BP_Blogs_Blog::get()'s use of wp_parse_id_list().
That function converted negative numbers over to 1.

Therefore, we need to pass a positive integer.  We're using the largest
positive integer available under bigint(20) to ensure no matches are found.

See #31.
Follow Blogs: Do not restrict activity filtering to users.

This is done by setting the 'user_id' parameter to 0.

See #31.
Follow Blogs: Fix WP adminbar addition.

See #31.
Rejig WP adminbar "Activity" subnav injection.
Follow Blogs: Add "Activity > Followed Blogs" to WP adminbar.

See #31.
Follow Blogs: Fix "Sites > Sites I Follow" WP adminbar subnav.

See #31.
Fix issue with member loop existence and follow user button defaults.
Namespace actions and hooks.

This is done to avoid any plugins using existing hooks and conflicting with
any third-party follow components.