Email product registrants access check is too strict

Problem/Motivation

Currently the "broadcast" route used by the "Email product registrants" page requires "administer registrations" permission. This means someone with both "Manage registrations" and "Manage sending registrant email" permissions is unable to use it, even though having both those permissions should be sufficient.

Steps to reproduce

Assign both the "Manage registrations" and "Manage sending registrant email" permissions to a user. Login as that user and attempt to access the "Email product registrants" page for a given commerce product, and observe the link is not present.

Proposed resolution

Fix the route requirements.

User interface changes

None

API changes

None

Data model changes

None