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/ModAPI.h

@@ -0,0 +1,12 @@
+#pragma once
+
+#include <QString>
+
+class ModAPI {
+    public:
+        virtual ~ModAPI() = default;
+
+        inline virtual QString getModSearchURL(int, QString, QString, bool, QString) const { return ""; };
+        inline virtual QString getVersionsURL(const QString& addonId) const { return ""; };
+        inline virtual QString getAuthorURL(const QString& name) const { return ""; };
+};