Support dynamic BuddyPress group forum activity actions

[johnjamesjacoby]

Since [6392], the PHPUnit tests for dynamic group forum topic and reply activity actions have remained skipped.

bbPress registers the bbp_topic_create and bbp_reply_create formatting callbacks for its own activity component, while group forum activity is stored under the BuddyPress groups component. BuddyPress therefore cannot dynamically rebuild those group activity actions, so their stored author, topic, and forum text can become stale.

Register or otherwise provide formatting for group-scoped forum activity without introducing duplicate activity-filter entries. Enable the existing tests once the behavior is supported.

Follow-up to #2794, #3089, and #3695.

[johnjamesjacoby]

In 7826:

Tests: Complete skipped-test audit.

Run URL assertions on multisite, where they now pass, and correct BuddyPress group test fixtures so nonce and capability paths execute in the intended request context. Document the remaining dynamic group activity limitation.

In trunk, for 2.7.

See #3705.
Fixes #3695.