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.

tailwind.config.js

@@ -3,19 +3,28 @@
 const colors = require('tailwindcss/colors')
 
 module.exports = {
+    variants: {
+        extend: {
+            backgroundOpacity: ['dark']
+        }
+    },
     purge: false,
     darkMode: 'class', // or 'media' or 'class'
     theme: {
         extend: {
             colors: {
                 bluegray: colors.blueGray,
+                warmgray: colors.warmGray,
                 amber: colors.amber,
-                teal: colors.teal
+                orange: colors.orange,
+                teal: colors.teal,
+                lime: colors.lime,
+                lightblue: colors.lightBlue
             }
         }
     },
-    variants: {},
     plugins: [
+        require('@tailwindcss/forms')
     ]
 //    prefix: 'tw-'
 }