First version of new ui based on tailwind

This drops fomantic-ui as css toolkit and introduces tailwindcss. With
tailwind there are no predefined components, but it's very easy to
create those. So customizing the look&feel is much simpler, most of
the time no additional css is needed.

This requires a complete rewrite of the markup + styles. Luckily all
logic can be kept as is. The now old ui is not removed, it is still
available by using a request header `Docspell-Ui` with a value of `1`
for the old ui and `2` for the new ui.

Another addition is "dev mode", where docspell serves assets with a
no-cache header, to disable browser caching. This makes developing a
lot easier.

package.json

@@ -4,6 +4,7 @@
   "private": true,
   "dependencies": {
     "@fortawesome/fontawesome-free": "^5.15.2",
+    "@tailwindcss/forms": "^0.2.1",
     "autoprefixer": "^10.2.4",
     "cssnano": "^4.1.10",
     "postcss": "^8.2.4",