This is in preparation for dealing with cross-wiki notifications
where we may need several types of operations to extract bundled
notifications from local and external APIs.
Also, renamed files:
* mw.echo.dm.AbstractAPIHandler -> mw.echo.dm.APIHandler
* mw.echo.dm.APIHandler -> mw.echo.dm.LocalAPIHandler
* All API-related handler files moved to their own folder
for better organization.
Change-Id: Ib730c780ea52c93a6026c5d0b22012b6f39bb50d
