_content/tour: handle PageUp/PageDown anywhere on a lesson page

[thevilledev]

The onpageup and onpagedown directives listened for keyup on
#editor-container only, so the keys stopped working as soon as the
focus moved outside of it. For example, to the theme toggle or the
"A Tour of Go" link in the top bar, or to the body after clicking on
empty space.

Listen on the document instead, as long as the editor view is
alive. Remove the handler when its scope is destroyed so that leaving
the lesson (e.g. for /tour/list) does not leave stale handlers behind.
Only handle the keys in the lesson, the top bar, or with nothing
focused, and keep ignoring them with Ctrl held or in editable fields
other than the code editor. Outside of the lesson, also leave them to
the browser when the page scrolls, as on narrow screens.

Fixes golang/go#81596

[gopherbot]

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