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_web/controllers/sources/source_html/new.html.heex

@@ -1,5 +1,5 @@
 <div class="mb-6 flex gap-3 flex-row items-center">
-  <.link :if={!Plug.Conn.get_session(@conn, :onboarding)} navigate={~p"/sources"}>
+  <.link :if={!Settings.get!(:onboarding)} navigate={~p"/sources"}>
     <.icon name="hero-arrow-left" class="w-10 h-10 hover:dark:text-white" />
   </.link>
   <h2 class="text-title-md2 font-bold text-black dark:text-white ml-4">New Source</h2>