Circumvent the max_input_vars limitation

Split out from #1032270: Menu not changed, no error message displayed (max_input_vars).

Sometimes, the "Power edit" form for a menu is so big that we hit PHP max_input_vars limit.
In #2950256: Warn if form submission exceeds max_input_vars limit. we introduce a warning message for this case.

But could we actually circumvent the limitation?
E.g. with some javscript magic?

Proposed solution

On attempted form submission, a jquery script collects all form data, and packs it into a hidden form element as json.
It also removes most of the other elements, to reduce the number of input variables.

On server side, we unpack the json data to populate the $_POST array.

This still leaves some open questions. But the basic idea can work.