Added OPML Endpoint for podcast rss feeds (#512)

* Added OPML Endpoint for podcast rss feeds

* changed opml route and added controller test for opml endpoint

* add copy opml feed button

* add copy opml feed button - correct url

* fix html indenting

* add indentation to opml

Co-authored-by: Kieran <kieran.eglin@gmail.com>

* use convention for unused controller params

Co-authored-by: Kieran <kieran.eglin@gmail.com>

* add test for opml_sources helper function

* change opml endpoint to be more inline with the other routes

---------

Co-authored-by: robs <git@robs.social>
Co-authored-by: Kieran <kieran.eglin@gmail.com>

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

@@ -1,6 +1,16 @@
 <div class="mb-6 flex gap-3 flex-row items-center justify-between">
   <h2 class="text-title-md2 font-bold text-black dark:text-white">Sources</h2>
   <nav>
+    <.button color="bg-transparent" x-data="{ copied: false }" x-on:click={~s"
+          copyWithCallbacks(
+            '#{opml_feed_url(@conn)}',
+            () => copied = true,
+            () => copied = false
+          )
+        "}>
+      Copy OPML Feed
+      <span x-show="copied" x-transition.duration.150ms><.icon name="hero-check" class="ml-2 h-4 w-4" /></span>
+    </.button>
     <.link href={~p"/sources/new"}>
       <.button color="bg-primary" rounding="rounded-lg">
         <span class="font-bold mx-2">+</span> New <span class="hidden sm:inline pl-1">Source</span>