BPGES: Update forum notifications to better work with HTML.

View corresponding commits:
- https://github.com/boonebgorges/buddypress-group-email-subscription/commit/09ae6c2c35a6a68869ad6af8164b79eaebc54e21
- https://github.com/boonebgorges/buddypress-group-email-subscription/commit/4509dab1af827eb5a4bd6dc283b671f1798a5ce8
BPGES: Add support for group activity items.

Requires bleeding version of BP Group Subscription plugin:
https://github.com/boonebgorges/buddypress-group-email-subscription/
Whitespace.
Minor code reorganization.