ui: Add friendly date to Mailbox message view

- Refactor some date stuff

ui/src/Data/Date.elm

@@ -0,0 +1,21 @@
+module Data.Date exposing (..)
+
+import Date exposing (Date)
+import Json.Decode as Decode exposing (..)
+
+
+{-| Decode an ISO 8601 date
+-}
+date : Decoder Date
+date =
+    let
+        convert : String -> Decoder Date
+        convert raw =
+            case Date.fromString raw of
+                Ok date ->
+                    succeed date
+
+                Err error ->
+                    fail error
+    in
+        string |> andThen convert