2021-August-12 - Prerelease - 2.6.321

Map Tags & Use Maps Server (triplea_maps.yml now unused) (#9491)

__Always use Maps Server__
  Stop reading 'triplea_maps.yml'
  Specifically instead of downloading the 'triplea_maps.yml'
  file and parsing it, the front end will instead contact
  the server and receive a JSON payload that contains the
  map download list.

__Introduce and use map tags__
   Map tags are a generic
  way to add additional meta data to maps dynamically.
  For example, instead of having a 'category' value
  specifically enumerated, we instead now have a
  tag whose key is 'category'. This allows us to have
  an arbitrary set of meta information without
  needing to update the front end.

__Update server to send 'preview image url' to the frontend__
  The server computes the preview image URL to make it easier
  and more explicit for the frontend. Having the server
  instead of the client compute the preview image URL also
  allows for it to be updated in database.