Support creating Trips from the frontend

*sigh* ... spent way too much time encoding/decoding date types...

I need my database, server, client, and JSON need to agree on types.

TL;DR:
- Add CSS for elm/datepicker library
- Create Common.allErrors to display UI errors
- Prefer Data.Time.Calendar.Day instead of newtype Date wrapper around Text

client/src/Common.elm

@@ -0,0 +1,27 @@
+module Common exposing (..)
+
+import Html exposing (..)
+import Maybe.Extra as ME
+import State
+import UI
+import Utils
+
+
+allErrors : State.Model -> Html State.Msg
+allErrors model =
+    div []
+        (State.allErrors
+            model
+            |> List.map
+                (\( mError, title ) ->
+                    case mError of
+                        Nothing ->
+                            text ""
+
+                        Just err ->
+                            UI.errorBanner
+                                { title = title
+                                , body = Utils.explainHttpError err
+                                }
+                )
+        )