Use a custom PlaceController to go to the previous page after login.

Use a custom PlaceController to go to the previous page after login.

        * src/main/java/org/glom/web/client/PlaceControllerExt.java:
A derived PlaceController that knows the previous, and default,
places, and can take us to the appropriate one.
        * src/main/java/org/glom/web/client/ClientFactoryImpl.java:
        Instantiate the custom PlaceController instead of the regular one.
        * src/main/java/org/glom/web/client/OnlineGlom.java:
Use the defaultPlace from the PlaceControllerExt, to avoid
duplication.
        * src/main/java/org/glom/web/client/activity/DocumentLoginActivity.java:
        Add goToPrevious() and goToDefault(), using the custom PlaceController.
        Use these instead of the hard-coded goTo() calls.