Allow users to use POST instead of GET #1

There should be a form at the bottom of the page to switch HTTP method used, and admins should be allowed to choose the default one.

b4ae9acb4f is fully replacing GET by POST for QR codes generation.

The choice is not left to the user or the admin, since

POST is considered more private (generally not logged by the server or the client) than GET
users never need to use GET anymore because the select input type of margin and size have been replaced by number input type, letting users choose exact value through the interface