Add 'offset' parameter to bp_has_activities() function stack

This param allows plugins and themes to filter results to only those activity
items whose IDs are greater than the 'offset' parameter. See eg the "load
new items" integration with the Heartbeat API.

See #5328

Props imath, boonebgorges

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7951 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Automatically check for newly created items when viewing the Activity Stream.

Leveraging WP's Heartbeat API, this new functionality performs periodic checks
to see whether new activity items matching the current filter have been posted
since the page was originally loaded. If new items are found, a Load Newest
link is inserted into the top of the stream, which will insert the new items.

A filter is included, bp_activity_heartbeat_pulse, that allows site owners to
set a specific interval for these checks. The default value is 15 seconds, or
whatever your global Heartbeat interval is.

To disable the feature, there is a new setting "Automatically check for new
activity items when viewing the activity stream".

Fixes #5328

Props imath, boonebgorges

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7952 cdf35c40-ae34-48e0-9cc9-0c9da1808c22