#64 Keyed URL for temporary access to a private section

[Jonathan Maybaum]

The goal is to have a way to generate a URL with an
embedded key that would allow the user to enter an
access-protected section, without needing to have an
account or be given explicit access permission. The key
would time out after a number of days specified by the
site owner at the time of key creation, using a default
value from app preferences. An option would be to allow
access to all sections in the site.

Maybe have a page to generate the key that shows all
sections, with checkboxes to choose the one(s) where
the key will work.

This is analogous to the existing functionality for
generating keyed URLs to get access-protected files.