[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/actions_dropdown.html.heex

@@ -26,6 +26,11 @@
       <span x-show="copied" x-transition.duration.150ms><.icon name="hero-check" class="ml-2 h-4 w-4" /></span>
     </span>
   </:option>
+  <:option>
+    <.link href={~p"/sources/new?template_id=#{@source}"} method="get">
+      Use as Template
+    </.link>
+  </:option>
   <:option>
     <div class="h-px w-full bg-bodydark2"></div>
   </:option>