In some cases multiple state modules may need to handle the same action.
For example, when annotations are loaded or unloaded we may want to
update both the set of loaded annotations and also the current
selection.

This commit changes the `createReducer` helper and the code that uses it
in `reducers/index.js` so that multiple state modules can define update
functions that handle the same action.

Note that all update functions get the same, _previous_ state as input
and there is no guarantee on precedence if they return conflicting
updates.

See also: https://redux.js.org/faq/actions#is-there-always-a-one-to-one-mapping-between-reducers-and-actions