Allow plugins to override default digest markup

[boonebgorges]

It's not currently possible to override the basic logic that BPGES uses to build digest HTML. There are no filters, except on single pieces of markup, and the way ass_digest_fire() is constructed mixes together business and display logic.

This should be improved, so that the function basically has three parts (which may be broken into separate functions):

1. The digest queue is pulled up and sorted
2. For each user who is to receive a digest, the HTML is built from the raw list of groups/activity_ids.
3. BPGES uses the HTML to send the digests

The function should be rewritten so that 2 can easily be done elsewhere.

[boonebgorges]

A note that the existing ass_digest_format_item_group and ass_digest_format_item filters can be leveraged for some of this work. But ass_digest_format_item_group doesn't receive enough information to be properly modified - it needs at least a list of activity ids.