Follow Blogs: Fix notice when no activity scope is set.
Invalidate follow count cache when a user is deleted.

See #36.
Tests: Force bp_is_network_activated() to return true.

If the 'buddypress' directory is symlinked, the bp_is_network_activated()
function does not return true.  See https://buddypress.trac.wordpress.org/ticket/5489.

In the meantime, force bp_is_network_activated() to always return true when
multisite is in use when unit testing.  This will ensure that BP Follow
will load its blogs module.

See #42.
Tests: Add follow and delete blog test.

See #42.
Blogs: Add delete blog routine.

When a blog is deleted, invalidate the blog count cache and removes follow
relationships for those that were following the blog.

See #31, #42.