Fix MutationObserver tracking and clearing upon pageModified

Pre-patch, we were losing references to previously-registered mutation observers when the translate instance was dropped along with the Page object on every pageModified event. This was causing multiple mutation observers to get registered on the same nodes triggering more and more detectWeb calls each time.
We are now careful to disconnect all mutation observers before redoing detection for the whole tab (i.e. all documents in page)
Re https://github.com/proquest/translators/pull/1