Add extended configuration support (w/ other miscellaneous fixes)

https://phabricator.apertis.org/T8854

Some important notes:

• Please, please review this. Some parts of qa-report-app are a bit...err, confusing, and I would very much appreciate all the testing this can get!
• Don't get too hung up on "platform" vs "type" vs "image"; the source code already uses these terms almost interchangeably but then, confusingly, also for different things? (Cleaning up some of this could maybe be an R&D task later on?) I chose "platform" here primarily since it seems to match the use in various other places in Apertis.
• As-is, this will log when it encounters test case configurations for platforms that don't actually exist. This results in...quite a few warnings, e.g.:

  # All our LXC platforms are 'public', not 'internal'
  Test case 'tiny-container-user-basic' uses invalid platform tiny-lxc-armhf-*-internal (release=v2022dev2)
  # We don't have arm64/armhf SDK images
  Test case 'flatdeb-demo' uses invalid platform sdk-arm64-*-public (release=v2022dev3)

  Is this...actually something we care about?