Converted template model to inherit from EmailMutiAlternatives
Added separate body template for html template
Added autogeneration of text content from HTML templates
Updated test fixture
Hid HTML-related form fields in admin when HTML messages are disabled by
settings
Cleaned up admin form
Dynamically show/hide admin fields for email type and autogeneration
Added tests for HTML/multipart messages
Added documentation for HTML templates and options
Fixed typo in HTML message detection
Fixed test settings