Variant improvements in pools & map command

[Pablete1234]

Adds an option for pools to specify a list of variants, the maps will try to load their variants if any are available, in order:

pools:
  halloween:
    # Other stuff omitted for clarity
    variants:
      - halloween
      - hallo
    maps:
      - Map A
      - Map B

If any of the maps Map A or Map B have a variant with id "halloween", it will load that instead. If it doesn't have one of those, it will fall-back to "hallo", and if all fails it will end up with the default variant of the map.

Variants now also show up in /map, and you can click to look thru them:

Additionally, added a has-variant conditional:

<if has-variant="halloween">
  <unless variant="halloween">
    <phase>development</phase>
  </unless>
</if>

If put in the global xml, this would automatically dev-phase all non-halloween variants, for maps that have a halloween variant.