Accept richer text from remote statuses

Support abbr, del, pre, blockquote, code, strong, b, em, i, and h1…h5
HTML elements in remote statuses, add corresponding CSS.

app/javascript/flavours/glitch/styles/components/status.scss

@@ -22,7 +22,7 @@
     margin: -3px 0 0;
   }
 
-  p {
+  p, pre, blockquote {
     margin-bottom: 20px;
     white-space: pre-wrap;
 
@@ -31,6 +31,33 @@
     }
   }
 
+  h1, h2, h3, h4, h5 {
+    margin-top: 20px;
+    margin-bottom: 20px;
+  }
+
+  h1, h2 {
+    font-weight: 500;
+    font-size: 18px;
+  }
+
+  h2 {
+    font-size: 16px;
+  }
+
+  blockquote {
+    margin-left: 20px;
+    color: $dark-text-color;
+  }
+
+  b, strong {
+    font-weight: 500;
+  }
+
+  em, i {
+    font-style: italic;
+  }
+
   a {
     color: $secondary-text-color;
     text-decoration: none;

app/lib/sanitize_config.rb

@@ -20,11 +20,13 @@ class Sanitize
     end
 
     MASTODON_STRICT ||= freeze_config(
-      elements: %w(p br span a),
+      elements: %w(p br span a abbr del pre blockquote code b strong i em h1 h2 h3 h4 h5),
 
       attributes: {
-        'a'    => %w(href rel class),
+        'a'          => %w(href rel class title),
         'span'       => %w(class),
+        'abbr'       => %w(title),
+        'blockquote' => %w(cite),
       },
 
       add_attributes: {
@@ -36,6 +38,7 @@ class Sanitize
 
       protocols: {
         'a'          => { 'href' => HTTP_PROTOCOLS },
+        'blockquote' => { 'cite' => HTTP_PROTOCOLS },
       },
 
       transformers: [