Change cvar usages to use CVarDef and define them in CCVars (#2250)

* Change cvar usages to use CVarDef and define them in CCVars

* Merge fixes

* Remove duplicate cvar registration

Content.IntegrationTests/Tests/Lobby/CharacterCreationTest.cs

@@ -38,9 +38,7 @@ namespace Content.IntegrationTests.Tests.Lobby
 
             await server.WaitAssertion(() =>
             {
-                var lobbyCvar = CCVars.GameLobbyEnabled;
-                serverConfig.SetCVar(lobbyCvar.Name, true);
-
+                serverConfig.SetCVar(CCVars.GameLobbyEnabled, true);
                 serverTicker.RestartRound();
             });