In footer, replace text "Mastodon" with logo (#7545)

app/javascript/styles/mastodon/footer.scss

@@ -26,5 +26,13 @@
         text-decoration: none;
       }
     }
+
+    img {
+      margin: 0 4px;
+      position: relative;
+      bottom: -1px;
+      height: 18px;
+      vertical-align: top;
+    }
   }
 }

app/views/layouts/public.html.haml

@@ -12,6 +12,6 @@
     - else
       %span.footer__domain= link_to site_hostname, root_path
     %span.powered-by
-      != t('generic.powered_by', link: link_to('Mastodon', 'https://joinmastodon.org'))
+      != t('generic.powered_by', link: link_to('https://joinmastodon.org') { image_tag asset_pack_path('logo_full.svg'), alt: 'Mastodon' })
 
 = render template: 'layouts/application'