Update TestRegisterView to check mail outbox

* Add missing plain-text email template
* Check sent email has the correct link in both plain-text and html
Update changelog
Check more of the verification url

* Check the protocol and domain are also included in the url
Use regex to check entire verify email url
Bump incuna-test-utils to v2.1.0; fix tests
Bump incuna-test-utils to v3.2.0
Refactor APIRequestTestCase

* Use incuna-test-utils BaseAPIRequestTestCase
Bump incuna-test-utils to v5.0.0

* Add Python2AssertMixin to APIRequestTestCase and RequestTestCase
Merge pull request #68 from incuna/validation-email-fix

Update TestRegisterView to check mail outbox