Django-user-management

* Move sending of verification emails into UserRegister view from VerifyEmailMixin.
* Add delete method to GetToken view.
* Return HTTP_201_CREATED and ok message from VerifyAccountView.

* Move `avatar` code to self-contained app so it does not break
when extra dependencies are not installed.

**Note: this is backward incompatible release.**
Avatar related code should now be imported from `api.avatar` namespace
instead of previous `api` namespace. An example `ProfileAvatar` class view
lives now at `user_management.api.avatar.views.ProfileAvatar`
(not `user_management.api.views.ProfileAvatar`).

* Bump required version of incuna_mail to 0.2

* Add labels to password serializers' fields.

* Add user avatar model mixin, serializer and endpoint.
**Requires djangorestframework>=2.3.13.**

* Ensure all urls accept a trailing slash.