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/Page.elm

@@ -2,6 +2,7 @@ module Page exposing
     ( Page(..)
     , fromUrl
     , goto
+    , hasSidebar
     , href
     , isOpen
     , isSecured
@@ -70,6 +71,18 @@ isSecured page =
             True
 
 
+{-| Currently, all secured pages have a sidebar, except UploadPage.
+-}
+hasSidebar : Page -> Bool
+hasSidebar page =
+    case page of
+        UploadPage _ ->
+            False
+
+        _ ->
+            isSecured page
+
+
 isOpen : Page -> Bool
 isOpen page =
     not (isSecured page)