VOL-42: Modeling for Volunteer Commendation

Adds upgrade/install code for creating new activity type and related custom fields.

The behavior of the `is_reserved` flag on the custom group (set in `CRM_Volunteer_Upgrader->installCommendationActivityType()`) depends on civicrm/civicrm-core#3669 (see [Jira issue](https://issues.civicrm.org/jira/browse/CRM-14990)).