Added external link icons to joinmastodon navbar buttons (#4368)

app/views/about/more.html.haml

@@ -29,7 +29,10 @@
             - else
               = link_to t('auth.login'), new_user_session_path, class: 'webapp-btn'
           %li= link_to t('about.about_this'), about_more_path
-          %li= link_to t('about.other_instances'), 'https://joinmastodon.org/'
+          %li
+            = link_to 'https://joinmastodon.org/' do
+              = "#{t('about.other_instances')}"
+              %i.fa.fa-external-link{ style: 'padding-left: 5px;' }
 
       .container.hero
         .heading

app/views/about/show.html.haml

@@ -33,7 +33,10 @@
             - else
               = link_to t('auth.login'), new_user_session_path, class: 'webapp-btn'
           %li= link_to t('about.about_this'), about_more_path
-          %li= link_to t('about.other_instances'), 'https://joinmastodon.org/'
+          %li
+            = link_to 'https://joinmastodon.org/' do
+              = "#{t('about.other_instances')}"
+              %i.fa.fa-external-link{ style: 'padding-left: 5px;' }
 
       .container.hero
         .floats