Everything: Rich text redux (#5625)

* lord save me

* UI/ChatBox: Use the new `defStyle` param for `RenderMarkup`

The previous iteration didn't work because `AddMessage` can't inherit
its color from the PushColor (since we're not doing actual tag stacks
anymore).

* rebase touchup

Content.Client/Message/RichTextLabelExt.cs

@@ -1,5 +1,6 @@
 using Robust.Client.UserInterface.Controls;
 using Robust.Shared.Utility;
+using Robust.Shared.Utility.Markup;
 
 namespace Content.Client.Message
 {
@@ -7,7 +8,7 @@ namespace Content.Client.Message
     {
         public static void SetMarkup(this RichTextLabel label, string markup)
         {
-            label.SetMessage(FormattedMessage.FromMarkup(markup));
+            label.SetMessage(Basic.RenderMarkup(markup));
         }
     }
 }