[Feat]:Add more customization abilities to the Sign-In Page in Enterprise mode

[BobLouderback]

When using Enterprise Mode, in the Branding section, I'd like to add links, change font styles, adjust image sizes and such using either HTML or Markdown Language in the Signup page area. Additionally, it would be great if the "Preview" feature actually previewed the actual changes being made before saving.

I've tried using Markdown and HTML, but it doesn't work.