Ensure backward compatibility for 1.x signup links

In 1.x, volunteers were directed to civicrm/volunteer/signup?reset=1&vid=1 to signup for volunteer project ID 1. On this page, they would supply their personal details as well as select the shift (role/time) they were interested in taking.

In 2.x, civicrm/volunteer/signup is still used, but for a slightly different purpose. In the new workflow, the user has already selected one or more volunteer opportunities (perhaps across different projects) to sign up for, and they need only enter in their personal details.

The 2.x version of this form no longer makes use of the vid URL parameter, so we can preserve backward compatibility redirecting the request to an interface where the user can select opportunities if the parameter was supplied. The link pattern for that interface is as follows: civicrm/vol/#/volunteer/opportunities?project=1&dest=event.