PLT-6890 Fix various scrolling issues (#6727)

* Fix various scrolling issues

* Move reaction scrolling to reaction list

* Handle scrolling when RHS opens

* Only run scroll update code when posts change

webapp/stores/scroll_store.jsx

@@ -1,24 +0,0 @@
-// Copyright (c) 2015 Mattermost, Inc. All Rights Reserved.
-// See License.txt for license information.
-
-import EventEmitter from 'events';
-
-const UPDATE_POST_SCROLL_EVENT = 'update_post_scroll';
-
-class ScrollStoreClass extends EventEmitter {
-    emitPostScroll() {
-        this.emit(UPDATE_POST_SCROLL_EVENT);
-    }
-
-    addPostScrollListener(callback) {
-        this.on(UPDATE_POST_SCROLL_EVENT, callback);
-    }
-
-    removePostScrollListener(callback) {
-        this.removeListener(UPDATE_POST_SCROLL_EVENT, callback);
-    }
-}
-
-var ScrollStore = new ScrollStoreClass();
-export default ScrollStore;
-