Separate user-specified and originating url on long form feedback

Before this change, the URL attribute on anonymous feedback is being used to store both user-specified and machine-determined URLs - user-specified URLs for long-form feedback, and machine-generated for all other types. This means that the URL can't be relied upon to be valid across all anonymous feedback.

This change
- moves the storage the user-specified links to a separate attribute.
- back-fills the URL field in the contacts database for long-form feedback

This is the first step in making URL a mandatory, validated input for anonymous feedback.

/cc @fatbusinessman