In market_source campaign validation, allow the use of 15-character campaign IDs

In market_source campaign validation, allow the use of 15-character campaign IDs.

This is accomplished by passing the $val through the `salesforce_management_api_get_sfid18()` function, if it is available.

Note: If the 15-character campaign ID passes validation (after being converted to the 18-character version), the 15-character campaign ID will remain in the webform submission records and will be present in the sync object. Salesforce appears to handle this gracefully.