message: Remove const enums
  Const enums are deprecated[1] and breaks the build after the update from
  `react-scripts-ts` to `react-scripts`.
  This PR has no breaking changes.
  [1]:
diff: @@ -4,7 +4,7 @@ import { MessageType } from '../types/MessageCenter';
  const DEFAULT_GROUP_ID = 'default';
  const DEFAULT_MESSAGE_TYPE = MessageType.ERROR;
  -const enum MessageCenterActionKeys {
  +enum MessageCenterActionKeys {
  ADD_MESSAGE = 'ADD_MESSAGE',
  REMOVE_MESSAGE = 'REMOVE_MESSAGE',
  MARK_MESSAGE_AS_READ = 'MARK_MESSAGE_AS_READ',
  