Merge branch '338380-content-editor-html' into 'master'

app/assets/javascripts/content_editor/constants.js

...ssets/javascripts/content_editor/extensions/html_marks.js

app/assets/javascripts/content_editor/extensions/image.js

...assets/javascripts/content_editor/extensions/reference.js

...assets/javascripts/content_editor/extensions/subscript.js

...sets/javascripts/content_editor/extensions/superscript.js

...assets/javascripts/content_editor/extensions/task_item.js

...assets/javascripts/content_editor/extensions/task_list.js

...ascripts/content_editor/services/create_content_editor.js

app/assets/javascripts/content_editor/services/mark_utils.js

...avascripts/content_editor/services/markdown_serializer.js

...ascripts/content_editor/services/serialization_helpers.js

spec/frontend/content_editor/services/mark_utils_spec.js

spec/frontend/fixtures/api_markdown.yml