Add a Title to Label (Text and Media) activity type

As a teacher
I want to be able to add a title to a label
So the title could be shown in the course main content area

Acceptance criteria:

• Teachers could add/edit title of 'label' activity types.
• The title for labels will be optional, not required.
• The title will contain some help text to inform the user is not required:
  "Label titles are optional. It will help structure your course and will be displayed as well in the course index when performing certain actions."
• In edit mode, the title and label icon will be shown in the main content area as any other activity type title.
• In view mode, the title will be shown, but no the label icon.

This improvement will help us to build the show label activities in the course index.

Proto: https://www.figma.com/proto/OfCvqK069Oz6Fvqq2xcZWI/Create-a-course?node-id=2592%3A32227&viewport=-660%2C1081%2C0.8157169222831726&scaling=scale-down-width&page-id=1422%3A2949&hide-ui=1

Designs: https://www.figma.com/file/OfCvqK069Oz6Fvqq2xcZWI/Create-a-course?node-id=1422%3A2949