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,
Use this.$el instead of the absolute selector
Use function call for FileList.pageSize
this.$el is not the same as $('#app-content')

That is why we use `this.$el.parent()` instead
Stub for pageSize, because now it is dynamically calculated
Even better - usage of this.$container instead of this.$el.parent()
Typo
Changed fileList.pageSize to function call
Merge pull request #11524 from nazar-pc/patch-1

Page size calculation based on real page height