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.

modules/webapp/src/main/elm/Util/Html.elm

@@ -1,6 +1,7 @@
 module Util.Html exposing
     ( KeyCode(..)
     , checkbox
+    , checkbox2
     , classActive
     , intToKeyCode
     , onClickk
@@ -40,6 +41,15 @@ checkbox flag =
         checkboxUnchecked
 
 
+checkbox2 : Bool -> Html msg
+checkbox2 flag =
+    if flag then
+        i [ class "fa fa-check-square font-thin" ] []
+
+    else
+        i [ class "fa fa-minus-square font-thin" ] []
+
+
 type KeyCode
     = Up
     | Down