Build UI builder with live svelte

[cibernox]

Close #59

• Add error feedback when saving an empty page or a page with incorrect template. Try creating a new page and hit "Create Draft Page" - nothing is displayed but it should alert the user which error is happening
• Switching from the code editor to the visual editor should not trigger a save action
• Add a query param to switch editors
• Remove the "Beacon CMS" from the preview header? It seems unnecessary now that the page builder is embedded
• Remove the "D | T | P" from the preview header? I'm not sure what that means
• Isolate the page stylesheet from the admin stylesheet. Since the page stylesheet is injected into the body it does conflict with the stylesheet on admin causing the admin layout to break so we need to apply a prefix to act as a namespace
• Only load the visual editor if page.template == heex
• Only update @page_builder (the AST) when changing tabs (clicking on Visual Editor button) instead of updating it on every keystroke in the code editor
• Do not crash when page format change from HEEx to Markdown and the template becomes invalid. The user should be able change it back to HEEx

[cibernox]