add InteractiveShell.display_page config

Sends all pager output to regular output instead of pager.

Pagers are registered via the already-existing show_in_pager hook, rather than clobbering `page.page` like we used to do in `install_payload_page`.

This splits the hook dispatch part of `core.page.page` into `core.page.page`, leaving just the 'display-in-pager' stuff in `core.page.pager_page`. That means the comment in the docstring can be addressed by moving everything but the hook-dispatch function into utils.page, if that's what we want.

Ping @fperez, @ellisonbg about moving the basic page utility to utils.page, now that it can be cleanly separated.
This is done in both `page` and `pager_page`. Can we lose one copy?