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/ItemDetail/View2.elm

@@ -0,0 +1,63 @@
+module Page.ItemDetail.View2 exposing (viewContent, viewSidebar)
+
+import Comp.Basic as B
+import Comp.ItemDetail
+import Comp.ItemDetail.EditForm
+import Comp.ItemDetail.Model
+import Comp.MenuBar as MB
+import Data.Flags exposing (Flags)
+import Data.ItemNav exposing (ItemNav)
+import Data.UiSettings exposing (UiSettings)
+import Html exposing (..)
+import Html.Attributes exposing (..)
+import Html.Events exposing (onClick)
+import Page.ItemDetail.Data exposing (..)
+import Styles as S
+
+
+viewSidebar : Bool -> Flags -> UiSettings -> Model -> Html Msg
+viewSidebar visible _ settings model =
+    div
+        [ id "sidebar"
+        , class S.sidebar
+        , class S.sidebarBg
+        , classList [ ( "hidden", not visible ) ]
+        ]
+        [ div
+            [ class S.header2
+            , class "font-bold mt-2"
+            ]
+            [ i [ class "fa fa-pencil-alt mr-2" ] []
+            , text "Edit Metadata"
+            ]
+        , MB.viewSide
+            { start =
+                [ MB.CustomElement <|
+                    B.secondaryBasicButton
+                        { label = ""
+                        , icon = "fa fa-expand-alt"
+                        , disabled = model.detail.item.state == "created"
+                        , handler = onClick (ItemDetailMsg Comp.ItemDetail.Model.ToggleOpenAllAkkordionTabs)
+                        , attrs =
+                            [ title "Collapse/Expand"
+                            , href "#"
+                            ]
+                        }
+                ]
+            , end = []
+            , rootClasses = "text-sm mb-3 "
+            }
+        , Html.map ItemDetailMsg
+            (Comp.ItemDetail.EditForm.view2 settings model.detail)
+        ]
+
+
+viewContent : ItemNav -> Flags -> UiSettings -> Model -> Html Msg
+viewContent inav _ settings model =
+    div
+        [ id "content"
+        , class S.content
+        ]
+        [ Html.map ItemDetailMsg
+            (Comp.ItemDetail.view2 inav settings model.detail)
+        ]