[reports-service] Introduce e-mail template
Summary:
This diff introduces HTML templates using Maud library, which allows to rendering HTML using Rust macros.
The templates will be used to render HTML e-mails for reports.
Also included some base HTML page layout (the <html>, <head>, <body> tags) along with some basic CSS styling.
Depends on D8980
Test Plan:
Created a temporary HTTP endpoint that renders HTML and opened it in the browser.
Screenshots of the final result in D8988
Reviewers: michal, patryk, jon, tomek
Reviewed By: michal, tomek
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D8983