admin: Render subject of admin emails

Use the provided template variables to render not only the body of the
emails, but also the subject.

Fix apyrgio/synnefo#265
admin: Add tests for email rendering
admin: Send emails through send_admin_email

This function will assert if the request is valid, render the email and
then use the requested sender address to send it.

Fix apyrgio/synnefo#177
admin: Replace send_email with send_admin_email

Remove any email handling from the actions/views of each model, since
the `send_admin_email` utility handles these transparently.
admin: Add "From:" label in the UI
admin: Add tests for send_admin_email
Merge branch 'feature-admin-emails' into feature-admin