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/Main.elm

@@ -3,7 +3,8 @@ module Main exposing (init, main)
 import Api
 import App.Data exposing (..)
 import App.Update exposing (..)
-import App.View exposing (..)
+import App.View
+import App.View2
 import Browser exposing (Document)
 import Browser.Navigation exposing (Key)
 import Data.Flags exposing (Flags)
@@ -72,7 +73,13 @@ viewDoc model =
                     Page.pageName model.page
     in
     { title = model.flags.config.appName ++ ": " ++ title
-    , body = [ view model ]
+    , body =
+        if model.flags.config.uiVersion == 2 then
+            App.View2.view model
+
+        else
+            [ App.View.view model
+            ]
     }