Moving dependencies from ext.echo.base to more specific modules

mediawiki.api and mediawiki.Uri are desktop-only modules that are
not actually needed by ext.echo.base. Moving the dependency
declarations to more specific modules so that ext.echo.base is
usable by mobile notifications. ResourceLoader is smart enough to
not double-load them, so there should be no performance difference.

Also removing unused 'cancel' message.

Change-Id: I9fa8c4501e038ded9125c3b43e5207c355af2db9