response: Implement ignored parameters with MutableJsonResponse class.

Creates `MutableJsonResponse` as a subclass of Django's `HttpResponse`
that we can modify for ignored parameters in the response content.

Updates responses to include `ignored_parameters_unsupported` in
the response data through `has_request_variables`. Creates unit
test for this implementation in `test_decorators.py`.

The `method` parameter processed in `rest_dispatch` is not in the
`REQ` framework, so for any tests that pass that parameter, assert
for the ignored parameter with a comment.

Updates OpenAPI documentation for `ignored_parameters_unsupported`
being returned in the JSON success response for all endpoints.
Adds detailed documentation in the error handling article, and
links to that page in relevant locations throughout the API docs.

For the majority of endpoints, the documentation does not include
the array in any examples of return values, and instead links to
the error handling page. The exceptions are the three endpoints
that had previously supported this return value. The changes note
and example for these endpoints is also used in the error
handling page.

api_docs/changelog.md

@@ -20,6 +20,16 @@ format used by the Zulip server that they are interacting with.
 
 ## Changes in Zulip 7.0
 
+**Feature level 167**
+
+* [All REST API endpoints](/api/rest-error-handling#ignored-parameters):
+  Implemented `ignored_parameters_unsupported` as a possible return value
+  in the JSON success response for all endpoints. This value is a array
+  of any parameters that were sent in the request by the client that are
+  not supported by the endpoint. Previously, unsupported parameters were
+  silently ignored, except in the subset of endpoints which already
+  supported this return value; see feature levels 111, 96 and 78.
+
 **Feature level 166**
 
 * [`POST /messages`](/api/send-message): Eliminated the undocumented