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/elm.json

@@ -6,6 +6,7 @@
     "elm-version": "0.19.1",
     "dependencies": {
         "direct": {
+            "CurrySoftware/elm-datepicker": "4.0.0",
             "elm/browser": "1.0.2",
             "elm/core": "1.0.5",
             "elm/html": "1.0.0",
@@ -19,6 +20,7 @@
             "elm-community/list-extra": "8.2.3",
             "elm-community/maybe-extra": "5.2.0",
             "elm-community/random-extra": "3.1.0",
+            "justinmimbs/date": "3.2.1",
             "krisajenkins/remotedata": "6.0.1",
             "ryannhg/date-format": "2.3.0"
         },