webui: convert sketch-app-shell to use tailwind

Doing the conversion from shadowDOM to tailwind starting with the
outermost elements in the page because shadowDOM hides dom nodes from
ancestor styles, which means tailwind styles don't apply to an element
if its ancestors use shadowDOM.

- add new SketchTailwindElement that disables shadowDOM for tailwind
- convert sketch-app-shell to inherit from SketchTailwindElement
- convert sketch-app-shell's CSS from shadowDOM to tailwind classes
diff --git a/webui/src/global.css b/webui/src/global.css
new file mode 100644
index 0000000..f1d8c73
--- /dev/null
+++ b/webui/src/global.css
@@ -0,0 +1 @@
+@import "tailwindcss";