Free the large page picture when the bigger page view closes #4

Tapping a page on the "Print this?" screen opens it bigger. Closing that view only hides it, so the large picture stays in the page and uses memory until a page is opened again. That matters on the older phones this screen has been made faster for.

Where to look: closePageView() in Source code/server/public/app.js. renderPageView() already empties the same area (pvStage) before drawing.

Done when: closing the view empties pvStage, and opening a page again still works, including the same page twice in a row.