Onboarding improvements (#76)

* Disabled pro modal during onboarding

* Restructured the way onboarding changes settings

* Added better upload date template placeholder

lib/pinchflat_web/controllers/pages/page_html/onboarding_checklist.html.heex

@@ -39,7 +39,7 @@
         </p>
         <p class="text-md text-bodydark">Feel free to add more Media Profiles or Sources in the meantime!</p>
         <div class="mt-8">
-          <.link href={~p"/"}>
+          <.link href={~p"/?onboarding=0"}>
             <.button color="bg-primary" rounding="rounded-full" disabled={not @sources_exist}>
               Let's Go <span class="font-bold mx-2">🚀</span>
             </.button>