AutosuggestTextarea: bind `this`, fix crash in older browsers

app/soapbox/components/autosuggest-textarea.tsx

@@ -157,7 +157,8 @@ class AutosuggestTextarea extends ImmutablePureComponent<IAutosuggesteTextarea>
     if (lastTokenUpdated && !valueUpdated) {
       return false;
     } else {
-      return super.shouldComponentUpdate!(nextProps, nextState, undefined);
+      // https://stackoverflow.com/a/35962835
+      return super.shouldComponentUpdate!.bind(this)(nextProps, nextState, undefined);
     }
   }