Add RSS subscribe links with icon across blog pages, footer, and head

- Add <.rss_icon> inline SVG component in core_components.ex
- Inject RSS <link> tags into <head> on blog pages for auto-discovery
- Show RSS icon next to blog title on index and tag pages
- Show RSS icon on post pages next to back-link
- Add Subscribe section in footer with links to both feeds
- Wire feed URL through blog controller (index, show, tag actions)

174 tests pass, credo clean.

app/lib/firehose_web/components/layouts/root.html.heex

@@ -41,6 +41,9 @@
       <% end %>
       <meta name="twitter:card" content={meta.twitter_card} />
     <% end %>
+    <%= for feed <- get_content_for(assigns, :feed_links) do %>
+      <link rel="alternate" type="application/rss+xml" title={feed.title} href={feed.rss_url} />
+    <% end %>
   </head>
   <body>
     {@inner_content}