FEATURE: allow advanced specification of excerpts for posts

Previously users could control excerpt with `<span class='excerpt'>`
in Markdown, this is somewhat limited for plugins that need to define this
across a section. This adds support for DIV as well

app/assets/javascripts/pretty-text/white-lister.js.es6

@@ -183,6 +183,7 @@ const DEFAULT_LIST = [
   "small",
   "span[lang]",
   "span.excerpt",
+  "div.excerpt",
   "span.hashtag",
   "span.mention",
   "strike",

spec/components/post_creator_spec.rb

@@ -270,6 +270,21 @@ describe PostCreator do
         }.to_not change { topic.excerpt }
       end
 
+      it 'supports custom excerpts' do
+        raw = <<~MD
+          <div class='excerpt'>
+          I am
+
+          a custom excerpt
+          </div>
+
+          testing
+        MD
+        post = create_post(raw: raw)
+
+        expect(post.excerpt).to eq("I am\na custom excerpt")
+      end
+
       it 'creates post stats' do
 
         Draft.set(user, 'new_topic', 0, "test")