Fix navigation page size handling

Closes #892 (closed) #902 (closed) #903 (closed)

• The default page size of 20 has been moved to a constant in js/config.js to be more explicit
• Hexadecimal, octal, non-integer or infinite page sizes and page numbers are no longer accepted anywhere
• The store's display/navigationPageSize is no longer set to an empty string when switching back to the default page size, but to an explicit number
• The store's display/navigationPageSize uses a default value of null, and is then set to the default page size after the URL gets parsed, to make it very clear when we are setting the page size for the first time.