Adjust packet size to prevent download blocking

Prevent download blocking by webhost abuse filters when packet size is too big (https://shopplugin.net/support/topic/digital-downloads-problems). 8192 is arbitrary.
Adjust default number of products per page

Adjust default number of products per page to a factor of default number of products per row.
Default 25 products per page, and 3 products per row will lead to white space on one page and orphaned products on the next. For example when number of products in store is 27.
Restored checkbox toggle functionality

Local rate and Conditional checkbox didn't toggle due to additonal <div>
Merge pull request #2817 from Just-m/master

Adjust packet size to prevent download blocking