Add custom pageSize parameter for category-next/loadCategoryProducts

[cewald]

Short description and why it's useful

Add a custom page-size parameter to category-next/loadCategoryProducts method to be able to use other initial- and next-page-limit on category product lists. For example if you wan't more or less than the default 50 items per page on initial load.

I also added a config value for the systemFilterNames to expand them via configs.

For example, this would be handy if you want to provide a select in list view which can change the items per page page when you don't want infinite-scrolling without changing any core-code.
Or if you like to load only 12 items per page initially or an amount which is dividable by 2 to don't have a single item on last line.

Which environment this relates to

Check your case. In case of any doubts please read about Release Cycle

• Test version (https://test.storefrontcloud.io) - this is a new feature or improvement for Vue Storefront. I've created branch from develop branch and want to merge it back to develop
• RC version (https://next.storefrontcloud.io) - this is a stabilisation fix for Release Candidate of Vue Storefront. I've created branch from release branch and want to merge it back to release
• Stable version (https://demo.storefrontcloud.io) - this is an important fix for current stable version. I've created branch from hotfix or master branch and want to merge it back to hotfix

Upgrade Notes and Changelog

• No upgrade steps required (100% backward compatibility and no breaking changes)
• I've updated the Upgrade notes and Changelog on how to port existing VS sites with this new feature

IMPORTANT NOTICE - Remember to update CHANGELOG.md with description of your change

Contribution and currently important rules acceptance

• I read and followed contribution rules