Accept richer text from remote statuses

Support abbr, del, pre, blockquote, code, strong, b, em, i, ul, ol, li and
h1 to h5 tags in remote statuses.

app/javascript/styles/mastodon/components.scss

@@ -705,10 +705,6 @@
 
   &.status__content--with-spoiler {
     white-space: normal;
-
-    .status__content__text {
-      white-space: pre-wrap;
-    }
   }
 
   .emojione {
@@ -717,7 +713,9 @@
     margin: -3px 0 0;
   }
 
-  p {
+  p,
+  pre,
+  blockquote {
     margin-bottom: 20px;
     white-space: pre-wrap;
 
@@ -726,6 +724,53 @@
     }
   }
 
+  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;
+  }
+
+  ul,
+  ol {
+    margin-left: 1em;
+  }
+
+  ul {
+    list-style-type: disc;
+  }
+
+  ol {
+    list-style-type: decimal;
+  }
+
   a {
     color: $secondary-text-color;
     text-decoration: none;

app/lib/sanitize_config.rb

@@ -34,11 +34,13 @@ 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 ul ol li),
 
       attributes: {
-        'a'    => %w(href rel class),
+        'a'          => %w(href rel class title),
         'span'       => %w(class),
+        'abbr'       => %w(title),
+        'blockquote' => %w(cite),
       },
 
       add_attributes: {
@@ -50,6 +52,7 @@ class Sanitize
 
       protocols: {
         'a'          => { 'href' => HTTP_PROTOCOLS },
+        'blockquote' => { 'cite' => HTTP_PROTOCOLS },
       },
 
       transformers: [