List assigned Activities on User Dashboard

This patch attempts to answer the question, "Can I quickly see all the outstanding activities assigned to me?" by adding a list of "Your Activities" to the User Dashboard, showing all activities assigned to the user and having status of "scheduled".

This patch is written for v. 4.1 and v. 3.4; it should work fine for both versions (and we're hoping to get this into the 3.4 branch for the sake of our own clients).

Adding code in 4.1 to allow this dashboard element to be enabled or disabled via Site Preferences (consistent with all other user dashboard sections).