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/Credits/CreditsWindow.xaml.cs

@@ -15,6 +15,7 @@ using Robust.Shared.IoC;
 using Robust.Shared.Localization;
 using Robust.Shared.Maths;
 using Robust.Shared.Utility;
+using Robust.Shared.Utility.Markup;
 using YamlDotNet.RepresentationModel;
 using static Robust.Client.UserInterface.Controls.BoxContainer;
 
@@ -143,7 +144,7 @@ namespace Content.Client.Credits
                 var text = _resourceManager.ContentFileReadAllText($"/Credits/{path}");
                 if (markup)
                 {
-                    label.SetMessage(FormattedMessage.FromMarkup(text.Trim()));
+                    label.SetMessage(Basic.RenderMarkup(text.Trim()));
                 }
                 else
                 {