Page size calculation based on real page height

This is fix for https://github.com/owncloud/core/issues/10060
Instead of hard coding page size as 20 items, we check real page height, and divide by 50 (height of one row).
This will allow to load fewer items on small screens and enough items on large screens (4k, portrait orientation, etc.).
Also checking page height on every load to respond on browser window resizing,
Please use `this.$el` instead of the absolute selector (I believe it should be the same element)