Use get_date_from_gmt() when displaying dates on Activity Management panel

Dates were previously showing up in UTC. The problem only arises in on this
Dashboard panel because elsewhere in BP, bp_core_time_since() is used for
activity timestamps, and the latter function does the necessary UTC conversion.

Fixes #4573

Props shanebp, trishasalas

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7240 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Adds shanebp to 1.8 props list

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7241 cdf35c40-ae34-48e0-9cc9-0c9da1808c22