Rewrite parsers to use AppImages

[pchote]

The different engine versions / parsers should be exposed through the admin interface, with a dropdown allowing the admin to select an AppImage from a defined directory and a second dropdown specifying the parser's grid type (to generalize the fix for OpenRA/OpenRA#15425). This would make it much easier and safer to update for new versions, and also to support third party mods.

Depends on OpenRA/OpenRA#15521.

[pchote]

#337 made a start towards this by cleaning up the version plumbing. The next step will be to migrate to a model, and then to integrate with the AppImages.

Moving to this model will require third party mods to explicitly request registration, with maps from unknown mods being rejected on upload. This adds a bit of hassle, but fixes the fundamental issues with mod grid type (OpenRA/OpenRA#15425) and will let us show previews that include proper spawn point and tech structure (OpenRA/OpenRA#3359) artwork, and automatically expose "official" maps (#295).