Try to unify our config files a bit

I've tried so many rewrites of the config system over the last few
months, in an attempt to get started on #1727. All of them stink, so
this is an attempt to apply some of the cleanup.

 - Move some of the common logic into ConfigFile. This means we now
   store more information ourselves for Forge, rather than reading it
   out of the ForgeConfigSpec.

 - Don't include the Range/Allowed keys in the translation key. This was
   mostly there because of how we read comments from Forge, but it never
   made much sense.

 - Remove our separate Trie structure, and just encode the tree as part
   of the children of a Group.