Add contingency plan page.

This adds a contingency plan page, linked from the FAQ for “will you
save my link forever?”.

This also does some cleanup for static pages:
- Check logged-in status via request.user variable accessible in every
template.
- Use generic template view for templates that don’t need special setup.
Merge branch 'dev.contingency' of https://github.com/jcushman/perma into jcushman-dev.contingency

Conflicts:
	perma_web/perma/templates/single-link.html
Merge branch 'develop' into jcushman-dev.contingency