Add twitter:card metatag to enable Twitter Cards support

app/views/about/index.html.haml

@@ -9,6 +9,7 @@
   %meta{ property: 'og:image', content: asset_url('mastodon_small.jpg') }/
   %meta{ property: 'og:image:width', content: '400' }/
   %meta{ property: 'og:image:height', content: '400' }/
+  %meta{ property: 'twitter:card', content: 'summary' }/
 
 .wrapper
   %h1

app/views/accounts/show.html.haml

@@ -12,6 +12,7 @@
   %meta{ property: 'og:image', content: full_asset_url(@account.avatar.url(:original)) }/
   %meta{ property: 'og:image:width', content: '120' }/
   %meta{ property: 'og:image:height', content: '120' }/
+  %meta{ property: 'twitter:card', content: 'summary' }/
 
 = render partial: 'header'
 

app/views/stream_entries/show.html.haml

@@ -14,5 +14,7 @@
     %meta{ property: 'og:image:width', content: '120' }/
     %meta{ property: 'og:image:height', content: '120' }/
 
+  %meta{ property: 'twitter:card', content: 'summary' }/
+
 .activity-stream.activity-stream-headless
   = render partial: @type, locals: { @type.to_sym => @stream_entry.activity, include_threads: true }