[reports-service] E-mail subject and summary text

Summary:
Added some content to the e-mail and a subject generation function that will be used later.

Depends on D8983

Test Plan:
Created a temporary HTTP endpoint that renders this HTML and opened it in the browser.
Screenshots of the final result in D8988

Reviewers: michal, patryk, jon

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8984