Digest enhancements

Hi Boone,

I've done a number of things for digests so it will perform a bit better.

* commit 3f5ed61 - Switch from `in_array()` and use `isset()`. (See #39)

* commit d0200e1 - Do not use `bp_activity_get_specific()`

* commit 4dfa4fe - Use an alternative function for `bp_core_get_user_domain()` to prevent using it in our digest foreach loop.

* commit 4f95d60 - Remove reply count from weekly digests as that uses `'bbpress_init'` for each forum topic.

* commit 61dc047 - Check if user's subscribed activity IDs still exist. (See #25)

Let me know what you think.

Also, @epawel, since you have a rather large BP site that relies on GES, if you can test these changes and let me know how it performs that would be great as well.