Clean up build email fixes. 

Send the notifications from the web celeries for now.
Clean up tasks.py a bit
Add start of base email template, add general multipart mailer

Adds multipart mailer and a semi-mocked up email example.
Fix up lingering email issues, some content changes
Slight change on build failure email
Add proper settings for syncers
Refactor tasks for double remote usage
Merge remote-tracking branch 'origin/fix-build-emails' into syncer-backend

Conflicts:
	readthedocs/core/utils.py
	readthedocs/projects/tasks.py