Repo structure for step-wise upgrades

[jernst]

Each release has a timestamp, let's call it TS.

• pacman.conf needs to generate to use it to identify the package db files, like this:

   [os-TS]
   https://depot.ubos.net/$channel/$arch/os
  

  This will lead to URL https://depot.ubos.net/$channel/$arch/os/os-TS.db

• There's a JSON file in the repo that lists the available versions in sequence, such as a JSON array. This is augmented when a new release is generated, and shortened when we purge things. (Maybe "purge" needs to decide what to purge based on the package db files)

• When generating pacman.conf during ubos-admin update, the TS to be used is the timestamp "after" the current timestamp.