'Load more' check should be based on retrieved data, not what could render
'Load more' check should be based on retrieved data, not what could render

Since EchoEventPresentationModel::canRender can reject notifications,
its result should no longer be used for infinite scroll.

Current code checks if the amount of to-be-rendered data exceeds what
we need to display, but if we reject some notifications that may no
longer be the case, even though there is more data still.

Change-Id: I3e5f8c2d1fc0c63db7b277324c96af043689ddce

wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo