Optimize and fix some bugs in indexing (#98)

* Improve curseforge and local indexing

This should make curseforge indexing more efficient, and reuses
some of the normal local indexing for the queued indexing of
recently created mods.

* Unify impls for single and multiple routes for mods and versions

This uses the same backend for the single and multiple query
routes so that they no longer return inconsistent information.

* Cache valid curseforge mod ids to reduce request load

This caches the ids of minecraft mods and reuses them on indexing
to reduce the amount of unused addons that are returned.

src/search/mod.rs

@@ -66,13 +66,13 @@ pub struct UploadSearchMod {
     pub author: String,
     pub title: String,
     pub description: String,
-    pub categories: Vec<String>,
+    pub categories: Vec<Cow<'static, str>>,
     pub versions: Vec<String>,
     pub downloads: i32,
     pub page_url: String,
     pub icon_url: String,
     pub author_url: String,
-    pub latest_version: String,
+    pub latest_version: Cow<'static, str>,
 
     /// RFC 3339 formatted creation date of the mod
     pub date_created: DateTime<Utc>,