Removed Jed from the headless build and instead let `__` call sprintf.
It's now up to downstream users for `@converse/headless` to decide
whether they want i18n support for the few translatable strings in that package
and to implement it themselves.
* declare picker events in lit-html
* init intersection observer only once
* don't set value manually
* don't manually add classes
* avoid x-scrollbar and 'undefined' in search input
