NOISSUE Use constants for user agents

Here lies yet another early-stage move to debrand the MultiMC codebase,
as well as reducing the burden of updating strings across the codebase
for a future MultiMC6.

buildconfig/BuildConfig.h

@@ -31,6 +31,11 @@ public:
     /// URL for the updater's channel
     QString CHANLIST_URL;
 
+    /// User-Agent to use.
+    QString USER_AGENT = "MultiMC/5.0";
+    /// User-Agent to use for uncached requests.
+    QString USER_AGENT_UNCACHED = "MultiMC/5.0 (Uncached)";
+
     /// Google analytics ID
     QString ANALYTICS_ID;