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/flame/FlameAPI.h

@@ -0,0 +1,27 @@
+#pragma once
+
+#include "modplatform/ModAPI.h"
+
+class FlameAPI : public ModAPI {
+   public:
+    inline QString getModSearchURL(int index, QString searchFilter, QString sort, bool fabricCompatible, QString version) const override 
+    {
+        return QString("https://addons-ecs.forgesvc.net/api/v2/addon/search?"
+            "gameId=432&" "categoryId=0&" "sectionId=6&"
+
+            "index=%1&"  "pageSize=25&"       "searchFilter=%2&"
+            "sort=%3&"   "modLoaderType=%4&"  "gameVersion=%5")
+                .arg(index)
+                .arg(searchFilter)
+                .arg(sort)
+                .arg(fabricCompatible ? 4 : 1)  // Enum: https://docs.curseforge.com/?http#tocS_ModLoaderType
+                .arg(version);
+    };
+
+    inline QString getVersionsURL(const QString& addonId) const override
+    {
+        return QString("https://addons-ecs.forgesvc.net/api/v2/addon/%1/files").arg(addonId);
+    };
+
+    inline QString getAuthorURL(const QString& name) const override { return ""; };
+};