Fix links in Volunteer tab to use Project ID rather than Event ID.
Put custom group name and custom activity type name in constants so they can
be accessed externally.
Volunteer activity type ID is no longer hardcoded in sample data.
Merge pull request #36 from fjg-conflux/CRM-12884

Fix links in Volunteer tab to use Project ID rather than Event ID.
Merge pull request #37 from fjg-conflux/schema_tweaks

Schema-ish tweaks