Initial backend test for user-based data/customizations (#4144)

Right now only support for colors and no real UX, idea is to test it & allow the idea to grow while figuring out the UX

tests/src/mocks/UserData.hpp

@@ -0,0 +1,27 @@
+#pragma once
+
+#include "controllers/userdata/UserDataController.hpp"
+
+namespace chatterino::mock {
+
+class UserDataController : public IUserDataController
+{
+public:
+    UserDataController() = default;
+
+    // Get extra data about a user
+    // If the user does not have any extra data, return none
+    boost::optional<UserData> getUser(const QString &userID) const override
+    {
+        return boost::none;
+    }
+
+    // Update or insert extra data for the user's color override
+    void setUserColor(const QString &userID,
+                      const QString &colorString) override
+    {
+        // do nothing
+    }
+};
+
+}  // namespace chatterino::mock