Fix serializeArrayKV for new jQuery

jQuery seems to return explicit undefined when it used to return null:
on form items without a value attribute.

jQuery also now returns null on selects that don’t have a value set so we can drop our check.
Use MAIN_DIR_FE, not MAIN_DIR_FE_PHP when linking webdir
Fix youtube links

they now get a protocol-relative url in the iframe tag and get_headers fails on these.