Add Authors to Activity/Project Readmes

[Bryan Wong]

Is your feature request related to a problem? Please describe.
It would be nice to give people credit for the curriculum they write and have it be known in the LMS.

Describe the solution you'd like
In the Activity/Project Readmes, include a field called contributors and have it be a list of Github usernames.

Example:

contributors

• poroia
• lazyplatypus
• wongband

On the backend create a new model called Author which has the github username.
Return data in Activity like:

authors: [
  {
    username: kavuong
  },
  {
    username: julma1040
  },
  {
    username: ryankuang1
  }
]