[facebook] Use `/me/taggable_friends` instead.

Facebook's API won't return a list of friends for users of the app, even
with the `user_friends` permission. It will only return friends who also use the
app.

We hack around this by using the `taggable_friends` API. It's worth noting that
this doesn't seem to return *all* friends, but most of them. I haven't looked
into why not all of them are returned, or what determines when a friend is, or
is not, added to the result list.

This is by far our slowest responding API and due to the oAuth issues described
noted in b814a378791d2c9e45608c88bd49757aa7ff63bc, it is possibly the hardest
API integrated as of yet, second only to Instagram.

Signed-off-by: Ricky Elrod <ricky@elrod.me>