refactor: move url creation for mods to modplatform/

Moves all things related to creating the URLs of the mod platforms
that go to network tasks to a single place, so that:

1. Maintaining and fixing eventual issues is more straightforward.
2. Makes it possible to factor out more common code between the
   different modplatform pages

launcher/modplatform/modrinth/ModrinthPackIndex.cpp

@@ -1,10 +1,13 @@
 #include "ModrinthPackIndex.h"
+#include "ModrinthAPI.h"
 
 #include "Json.h"
 #include "minecraft/MinecraftInstance.h"
 #include "minecraft/PackProfile.h"
 #include "net/NetJob.h"
 
+static ModrinthAPI api;
+
 void Modrinth::loadIndexedPack(ModPlatform::IndexedPack& pack, QJsonObject& obj)
 {
     pack.addonId = Json::requireString(obj, "project_id");
@@ -17,7 +20,7 @@ void Modrinth::loadIndexedPack(ModPlatform::IndexedPack& pack, QJsonObject& obj)
 
     ModPlatform::ModpackAuthor modAuthor;
     modAuthor.name = Json::requireString(obj, "author");
-    modAuthor.url = "https://modrinth.com/user/" + modAuthor.name;
+    modAuthor.url = api.getAuthorURL(modAuthor.name);
     pack.authors.append(modAuthor);
 }