Remove markdown tags in notifications' excerpts

Use a render that strips the markdown tags. This way we show a more
understandable excerpt in the list of notifications.

src/api/app/presenters/notification_presenter.rb

@@ -1,3 +1,5 @@
+require 'redcarpet/render_strip'
+
 class NotificationPresenter < SimpleDelegator
   TRUNCATION_LENGTH = 100
   TRUNCATION_ELLIPSIS_LENGTH = 3 # `...` is the default ellipsis for String#truncate
@@ -42,7 +44,7 @@ def excerpt
             when 'Review'
               @model.notifiable.reason
             when 'Comment'
-              @model.notifiable.body
+              render_without_markdown(@model.notifiable.body)
             else
               ''
             end
@@ -70,4 +72,12 @@ def avatar_objects
       @model.notifiable.reviews.in_state_new.map(&:reviewed_by) + User.where(login: @model.notifiable.creator)
     end
   end
+
+  private
+
+  def render_without_markdown(content)
+    # Initializes a Markdown parser, if needed
+    @remove_markdown_parser ||= Redcarpet::Markdown.new(Redcarpet::Render::StripDown)
+    ActionController::Base.helpers.sanitize(@remove_markdown_parser.render(content.to_s))
+  end
 end