Add nightly to the list of snapshots
Add nightly to the list of snapshots

I also cleaned up the LTS code. It currently had the assumption that LTS
major versions would be monotonically increasing from 0 without gaps.
While likely to be true, that's slightly brittle, and did in fact break
in my testing (where I only had an lts-2.4 in the database).

fpco/stackage-server