Allow generation of gdpr report from the email changelist.
Previously, this was only doable from the account changelist, but the main search criteria for gdpr requests is email address; so it's cumbersome to first search for an email address, grab e.g. the openid to do a search in the account changelist, and then build the gdpr report there.
The tests try not to duplicate things that are already tested in the account gdpr report tests.
Merged from https://code.launchpad.net/~roadmr/canonical-identity-provider/gdpr-report-from-email/+merge/368565