Ensure that users cannot set price on line items through the API
Fix format of price input in products/new.html.erb
[frontend] Stop people encountering uninitialized Taxon errors when they override ProductsController#show
Remove regression tests for #3404

These were proving extremely difficult to test, and so I have removed them. This issue should no longer come up, anyway.
[api] Remove 'add line items' test from orders controller

if people want to add line items, they should do it through the line items endpoint