Use content editor's markdown serializer for copy_as_gfm feature

Currently we have two instances of markdown serializers in our codebase:

• In content editor: app/assets/javascripts/content_editor/services/markdown_serializer.js
• In the copy_as_gfm feature: app/assets/javascripts/behaviors/markdown/serializer.js

By using the content editor's serializer for the copy_as_gfm feature, we can reduce our bundle size and delete a lot of redundant code. Additionally, content editor's serializer is very well tested and covers a lot more corner cases than the old one.