ui: Initial Elm UI import

Merged from https://github.com/jhillyerd/inbucket-elm

Uses https://github.com/halfzebra/create-elm-app

ui/src/Data/MessageHeader.elm

@@ -0,0 +1,29 @@
+module Data.MessageHeader exposing (..)
+
+import Json.Decode as Decode exposing (..)
+import Json.Decode.Pipeline exposing (..)
+
+
+type alias MessageHeader =
+    { mailbox : String
+    , id : String
+    , from : String
+    , to : List String
+    , subject : String
+    , date : String
+    , size : Int
+    , seen : Bool
+    }
+
+
+decoder : Decoder MessageHeader
+decoder =
+    decode MessageHeader
+        |> required "mailbox" string
+        |> required "id" string
+        |> optional "from" string ""
+        |> required "to" (list string)
+        |> optional "subject" string ""
+        |> required "date" string
+        |> required "size" int
+        |> required "seen" bool