Support CRUDing records on Admin page

TL;DR:
- Prefer the more precise verbiage, "Accounts", to "Users"
- Add username field to Trip instead of relying on session.username
- Ensure that decodeRole can JD.fail for invalid inputs

client/src/User.elm

@@ -89,7 +89,7 @@ renderTrip trip =
         , UI.wrapNoPrint
             (UI.textButton
                 { label = "Delete"
-                , handleClick = State.AttemptDeleteTrip trip.destination trip.startDate
+                , handleClick = State.AttemptDeleteTrip trip
                 }
             )
         ]