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.

src/main.rs

@@ -201,6 +201,8 @@ async fn main() -> std::io::Result<()> {
         });
     }
 
+    scheduler::schedule_versions(&mut scheduler, pool.clone(), skip_initial);
+
     let allowed_origins = dotenv::var("CORS_ORIGINS")
         .ok()
         .and_then(|s| serde_json::from_str::<Vec<String>>(&s).ok())