Game Version types, indexing, and bugfixes (#91)

* Add types to game_versions, allow filtering by version type

- Fixes an issue with version numbers in the initial mod indexing
  queue
- Modifies the /api/v1/categories/game_versions route to take an
  optional query parameter `type` to filter the listed game versions
- Creating tags is now idempotent
- Creating game_versions now requires a JSON body that specifies
  the version type

* Implement automatic indexing of new Minecraft versions

It's currently set to run every 6 hours and isn't configurable; we
could add config for it, but it doesn't seem likely to be rate
limited or have issues with frequency.

migrations/20201021214908_extend-game-version.sql

@@ -0,0 +1,3 @@
+
+ALTER TABLE game_versions
+ADD COLUMN type varchar(16) NOT NULL DEFAULT 'other';