overlay: Blur focused element while overlay is open.

We blur the focused element when an overlay opens, and refocus
it when the overlay is closed, to prevent side effects.

What motivated this change was that opening a lightbox overlay
from preview content while editing a message caused the escape
key to close the message edit form instead of closing the overlay.

web/src/overlays.ts

@@ -111,7 +111,9 @@ export function open_overlay(opts: OverlayOptions): void {
             reset_state();
         },
     };
-
+    if (document.activeElement) {
+        $(document.activeElement).trigger("blur");
+    }
     overlay_util.disable_scrolling();
     opts.$overlay.addClass("show");
     opts.$overlay.attr("aria-hidden", "false");