Rewrite cache decorator to reference args by name

`@cache.delete('user', 'user_id')` is easier to read and understand than
`@cache.delete('user', key_from_args=[1])`. This will become even more
apparent if we have to start doing stuff like `key_from_args=[1, 5]`,
which is a lot more opaque than just saying
`'service_id', 'template_id'`.

It does make the implementation a bit more complex, but I’m not too
worried about that because:
- the tests are solid
- it’s nicely encapsulated

app/notify_client/user_api_client.py

@@ -39,7 +39,7 @@ class UserApiClient(NotifyAdminAPIClient):
     def get_user(self, user_id):
         return User(self._get_user(user_id)['data'], max_failed_login_count=self.max_failed_login_count)
 
-    @cache.set('user')
+    @cache.set('user', 'user_id')
     def _get_user(self, user_id):
         return self.get("/user/{}".format(user_id))
 
@@ -61,7 +61,7 @@ class UserApiClient(NotifyAdminAPIClient):
             users.append(User(user, max_failed_login_count=self.max_failed_login_count))
         return users
 
-    @cache.delete('user')
+    @cache.delete('user', 'user_id')
     def update_user_attribute(self, user_id, **kwargs):
         data = dict(kwargs)
         disallowed_attributes = set(data.keys()) - ALLOWED_ATTRIBUTES
@@ -75,20 +75,20 @@ class UserApiClient(NotifyAdminAPIClient):
         user_data = self.post(url, data=data)
         return User(user_data['data'], max_failed_login_count=self.max_failed_login_count)
 
-    @cache.delete('user')
+    @cache.delete('user', 'user_id')
     def reset_failed_login_count(self, user_id):
         url = "/user/{}/reset-failed-login-count".format(user_id)
         user_data = self.post(url, data={})
         return User(user_data['data'], max_failed_login_count=self.max_failed_login_count)
 
-    @cache.delete('user')
+    @cache.delete('user', 'user_id')
     def update_password(self, user_id, password):
         data = {"_password": password}
         url = "/user/{}/update-password".format(user_id)
         user_data = self.post(url, data=data)
         return User(user_data['data'], max_failed_login_count=self.max_failed_login_count)
 
-    @cache.delete('user')
+    @cache.delete('user', 'user_id')
     def verify_password(self, user_id, password):
         try:
             url = "/user/{}/verify/password".format(user_id)
@@ -118,7 +118,7 @@ class UserApiClient(NotifyAdminAPIClient):
         endpoint = '/user/{0}/email-already-registered'.format(user_id)
         self.post(endpoint, data=data)
 
-    @cache.delete('user')
+    @cache.delete('user', 'user_id')
     def check_verify_code(self, user_id, code, code_type):
         data = {'code_type': code_type, 'code': code}
         endpoint = '/user/{}/verify/code'.format(user_id)
@@ -154,20 +154,20 @@ class UserApiClient(NotifyAdminAPIClient):
         resp = self.get(endpoint)
         return [User(data) for data in resp['data']]
 
-    @cache.delete('service')
-    @cache.delete('user', key_from_args=[1])
+    @cache.delete('service', 'service_id')
+    @cache.delete('user', 'user_id')
     def add_user_to_service(self, service_id, user_id, permissions):
         # permissions passed in are the combined admin roles, not db permissions
         endpoint = '/service/{}/users/{}'.format(service_id, user_id)
         data = [{'permission': x} for x in translate_permissions_from_admin_roles_to_db(permissions)]
         self.post(endpoint, data=data)
 
-    @cache.delete('user', key_from_args=[1])
+    @cache.delete('user', 'user_id')
     def add_user_to_organisation(self, org_id, user_id):
         resp = self.post('/organisations/{}/users/{}'.format(org_id, user_id), data={})
         return User(resp['data'], max_failed_login_count=self.max_failed_login_count)
 
-    @cache.delete('user')
+    @cache.delete('user', 'user_id')
     def set_user_permissions(self, user_id, service_id, permissions):
         # permissions passed in are the combined admin roles, not db permissions
         data = [{'permission': x} for x in translate_permissions_from_admin_roles_to_db(permissions)]
@@ -191,7 +191,7 @@ class UserApiClient(NotifyAdminAPIClient):
         else:
             return user
 
-    @cache.delete('user')
+    @cache.delete('user', 'user_id')
     def _activate_user(self, user_id):
         return self.post("/user/{}/activate".format(user_id), data=None)