Support a preparing status for meetups.

Adding a "Coming Soon" section to the city navigation where preparing cities are listed. Cities that are preparing also have a little notice on their pages.

(It might be better to just move the notice to the Markdown instead of having two statuses. I didn't see a way to access the YAML from the homepage.)

Also, if this is merged, whoever does so gets one free punch for me including whitespace and refactoring in a feature pull request.

This fixes #71.