Fix password context

Fixes the following error:
```
admin_1      | [2018-11-09 09:44:10,533] ERROR in app: Exception on /internal/auth/email [GET]
admin_1      | Traceback (most recent call last):
admin_1      |   File "/usr/lib/python3.6/site-packages/flask/app.py", line 2292, in wsgi_app
admin_1      |     response = self.full_dispatch_request()
admin_1      |   File "/usr/lib/python3.6/site-packages/flask/app.py", line 1815, in full_dispatch_request
admin_1      |     rv = self.handle_user_exception(e)
admin_1      |   File "/usr/lib/python3.6/site-packages/flask/app.py", line 1718, in handle_user_exception
admin_1      |     reraise(exc_type, exc_value, tb)
admin_1      |   File "/usr/lib/python3.6/site-packages/flask/_compat.py", line 35, in reraise
admin_1      |     raise value
admin_1      |   File "/usr/lib/python3.6/site-packages/flask/app.py", line 1813, in full_dispatch_request
admin_1      |     rv = self.dispatch_request()
admin_1      |   File "/usr/lib/python3.6/site-packages/flask/app.py", line 1799, in dispatch_request
admin_1      |     return self.view_functions[rule.endpoint](**req.view_args)
admin_1      |   File "/usr/lib/python3.6/site-packages/flask_limiter/extension.py", line 544, in __inner
admin_1      |     return obj(*a, **k)
admin_1      |   File "/app/mailu/internal/views/auth.py", line 18, in nginx_authentication
admin_1      |     headers = nginx.handle_authentication(flask.request.headers)
admin_1      |   File "/app/mailu/internal/nginx.py", line 48, in handle_authentication
admin_1      |     if user.check_password(password):
admin_1      |   File "/app/mailu/models.py", line 333, in check_password
admin_1      |     context = User.pw_context
admin_1      | AttributeError: type object 'User' has no attribute 'pw_context'
```

core/admin/mailu/models.py

@@ -330,7 +330,7 @@ class User(Base, Email):
         )
 
     def check_password(self, password):
-        context = User.pw_context
+        context = self.get_password_context()
         reference = re.match('({[^}]+})?(.*)', self.password).group(2)
         result = context.verify(password, reference)
         if result and context.identify(reference) != context.default_scheme():