Use entityRef widgets for activity contact filters on the volunteer report

Currently, filtering by activity contact – i.e., assigner (source), volunteer (assignee), and beneficiary (target) – is done by comparing a supplied string with the contacts' sort names. This is probably not very efficient, it is not very precise (it is difficult to be sure that the correct John Smith – or only one John Smith – is included in the report), and it is a little out of date with CiviCRM UX trends.

Change these fields to use entityref widgets instead.