MAJOR: see notes

This is _basically_ an upstream merge from
https://github.com/CPHDH/plugin-MobileJson/tree/2dcde9a500b15169d334ab21
b3840469b58ecd5d though with two new lines in
helpers/ItemJsonifier.php, adding “visibility” and “thumbnail” to the
output. Doing this merge the proper way was causing conflicts due to
the difference in directory structures.