mailutils: Flask-Email initial port
- Adds new configuration variable CFG_MAIL_BACKEND. (closes #1531)
- Adds html and plaintext email templates.
- Adds unit tests for sending emails.
- Adds mailtutils_backend_adminonly for sending emails only to CFG_SITE_ADMIN_EMAIL.
Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>