Update to incuna mail v2.0.0

* Use plain text email by default
Expose verification email subject template

* Set EMAIL_SUBJECT_TEMPLATE on VerifyEmailMixin
* Refactor send_validation_email to allow easier testing
Refactor send_validation_email

* Add _get_email_context helper method
* Simplify _get_email_kwargs method
Refactor test_send_validation_email

* Avoid duplicating work in test__email_context and
  test__get_email_kwargs
Expose email templates on VerifyEmailMixin

* Allow the email templates used by send_validation_email to be
  overriden or disabled.
* Allow the context provided to the email templates to be extended.
* Add a docstring to send_validation_email.
Rename _get_email_kwargs -> email_kwargs

* Make email_kwargs part of the public api
* Standardise naming to match email_context
Add get_email_subject method
Distinguish EMAIL_SUBJECT from django templates
Remove unnecessary variable in email_kwargs
Merge pull request #62 from incuna/verification-email

Allow more customisation of account verification email