Add settings model (#54)

* Adds a basic settings model

* Added more settings methods; Hooked up initial settings runner to app boot

* Update onboarding flow to use settings model instead of session data

lib/pinchflat/settings/setting.ex

@@ -0,0 +1,24 @@
+defmodule Pinchflat.Settings.Setting do
+  @moduledoc """
+  A Setting is a key-value pair with a datatype used to track user-level settings.
+  """
+
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  schema "settings" do
+    field :name, :string
+    field :value, :string
+    field :datatype, Ecto.Enum, values: ~w(boolean string integer float)a
+
+    timestamps(type: :utc_datetime)
+  end
+
+  @doc false
+  def changeset(setting, attrs) do
+    setting
+    |> cast(attrs, [:name, :value, :datatype])
+    |> validate_required([:name, :value, :datatype])
+    |> unique_constraint([:name])
+  end
+end