Add per-user configurable top-level tab hiding

[neggles]

hi fren, it's your worst nightmare, I actually did the thing! and it even accounts for dynamic/extension-added tabs!

well okay it only sort of works. by which I mean it does not work.

the magicks for dropdown popovers appear to be broken with select2's multi-select (since nothing else seems to ever actually call makeMultiselectInput()?) and I am not enough of a JS person to work out how to fix that. So the multiselect shows up, but it's styled wrong, and changing the selected options doesn't mark the option as altered so it's not saveable.

But the hiding does work if I manipulate things in the browser debugger to pretend like the server returned a list of tab IDs to hide.

what do, how fix?