Move all polls processing to events-sdk & prep for stable polls (#7517)

* Move all polls processing to events-sdk

This makes polls support the full range of extensible events (both parsing and generation).

* Appease the linter

* Fix & update tests

* Update events-sdk for polls bugfix

* Update events-sdk for typechecking

* Add missing type cast

* Update per review

src/components/views/messages/MessageEvent.tsx

@@ -17,8 +17,8 @@ limitations under the License.
 import React, { createRef } from 'react';
 import { EventType, MsgType } from "matrix-js-sdk/src/@types/event";
 import { Relations } from 'matrix-js-sdk/src/models/relations';
-import { POLL_START_EVENT_TYPE } from "matrix-js-sdk/src/@types/polls";
 import { LOCATION_EVENT_TYPE } from 'matrix-js-sdk/src/@types/location';
+import { M_POLL_START } from "matrix-events-sdk";
 
 import * as sdk from '../../../index';
 import SettingsStore from "../../../settings/SettingsStore";
@@ -125,7 +125,7 @@ export default class MessageEvent extends React.Component<IProps> implements IMe
                 BodyType = UnknownBody;
             }
 
-            if (type && type === POLL_START_EVENT_TYPE.name) {
+            if (M_POLL_START.matches(type)) {
                 // TODO: this can all disappear when Polls comes out of labs -
                 // instead, add something like this into this.evTypes:
                 // [EventType.Poll]: "messages.MPollBody"