External href links are death traps

Clicking a link changes the current window's location. This may or may not be desirable, but once you're there, there's no "Back to slides" button (and keyboard shortcuts for back don't work).

Reproduce the issue by opening an external href from a presentation and inspect the navigation and keyboard-shortcut handling entry points. Done means users can return to the slides after following an external link, either through a visible back control or a working back shortcut.

Written by the indexing model from the issue text.