Feature/fmo/multipart email

Add in support for multipart emails (html) in the automatic messages that respond to user actions. Eg: account creation, password change, ...

This PR:
- Adds a new send_mail function with the support for html
- Adds all the required templates to render the html messages (the base template is the one from the bulk_email app which ultimately is the one from the mailchimp templates)
- replaces the user.email_user function with the aforementioned send_mail
- Overrides the save method in class PasswordResetFormNoActive to use the new send_mail
Here I'm am overriding the method to add the additional functionality of supporting the 'html_email_template_name' arg. I could guess the name using the 'email_template_name' and adding a prefix or suffix, but that does not look like a good idea.