'/problem_reports' => '/anonymous_feedback/problem_reports'

ie to create a new problem report:

    POST /anonymous_feedback/problem_reports

this is because other types of anonymous feedback are about to
be added/split out:

    POST /anonymous_feedback/satisfaction_surveys
    POST /anonymous_feedback/
    POST /anonymous_feedback/page_not_found_reports

NOTE: the old endpoint (`/problem_reports`) is still in place, and can be removed once
all clients of the API are using the new endpoint