Make footer sticky (fixes #1)

Remaining problem: Resizing the window leaves an ugly whitespace where
to scrollbar should be.
Minor Refactoring

 - move session secret to config.json
 - allow configuration of hash iterations
 - use i18n instead of hardcoded string in ToS header
Add stub "About", "Help" and "Legal" pages (fixes #1)

Foundation Tabs are used for the subsections of the individual pages,
e.g. the ToS on the "Legal" page. As a consequence, URLs now use hashes
rather than backslashes.

This is rather ugly and should be fixed in the future (see issue #9).