Proposal: Hide sprint list and require a secret URL to access a sprint

For public use it might be nice to hide the public sprint list (so not just anyone can get into your sprint), and require a secret URL to join it (similar to https://www.shortreckonings.com/). We could:

* Move the "new sprint" form to the home page and remove the name validation.
* Assign _both_ an ID and name to a sprint. The ID would appear in the URL, whereas the name would only be visible on the sprint detail page.
* Delete the sprint list from the home page.

It might be good to think about this around the same time as #23 (DB persistence).

Start by reviewing this proposal alongside #23 (DB persistence) and inspect the current home page, sprint-list, and sprint-detail entry points. Define how sprint IDs, names, and secret URLs should interact. Done should mean sprint creation from the home page, no public sprint list, and access through the secret URL.

Written by the indexing model from the issue text.