NOISSUE Specify support URLs at build time

Support URLs (bug tracker, Discord guild, subreddit) are now specified
as cache variables in cmake, and the buttons are not shown if no value
is set for them.

This is an early-stage move towards debranding the MultiMC codebase,
and will (hopefully) alleviate support requests coming to us from
illicit forks.

buildconfig/BuildConfig.h

@@ -65,6 +65,10 @@ public:
      */
     QString META_URL;
 
+    QString BUG_TRACKER_URL;
+    QString DISCORD_URL;
+    QString SUBREDDIT_URL;
+
     QString RESOURCE_BASE = "https://resources.download.minecraft.net/";
     QString LIBRARY_BASE = "https://libraries.minecraft.net/";
     QString SKINS_BASE = "https://crafatar.com/skins/";