[Enhancement] Improve flow for adding many sources at once (#306)

* Added mechamism for using existing sources as a reference for adding new sources

* Added test

lib/pinchflat_web/controllers/sources/source_html/edit.html.heex

@@ -10,7 +10,12 @@
 <div class="rounded-sm border border-stroke bg-white px-5 pb-2.5 pt-6 shadow-default dark:border-strokedark dark:bg-boxdark sm:px-7.5 xl:pb-1">
   <div class="max-w-full">
     <div class="flex flex-col gap-10">
-      <.source_form changeset={@changeset} media_profiles={@media_profiles} action={~p"/sources/#{@source}"} />
+      <.source_form
+        changeset={@changeset}
+        media_profiles={@media_profiles}
+        action={~p"/sources/#{@source}"}
+        method="patch"
+      />
     </div>
   </div>
 </div>