Add 'highlighted' label to course index

As a user 
I want to see the highlighted section in the course index
So I can locate in the most important section of the course quickly

Acceptance criteria:

• If there is any highlighted section in the course, a 'highlighted' label is shown beside the section title in the course index for Boost theme.
• The current week is marked with a 'current' label in the course index for the Weeks format.

The UI and design of the labels would be similar to the design added as screenshot, but this issue is not about styling them. Making the design exact to the prototype could be done in MDL-72456 or any further issue.