Support contribution page ID in views contextual filter

CRM-15543 added the ability to have CiviCRM IDs exposed to Drupal Views contextual filters.

We have a requirement now to extract contribution page IDs, so here's a PR that adds support for extracting a contribution page ID from the URL.

Note: this only works on the 'first' hit of the contribution page form. After that the contribution page ID is gone from the URL and stored in the session in Civi.