Project details page "join the project" section API

[kristinashu]

Need api so members can "join the project"

Front end is happening in - Project details page "join the project" section #160

It should look like https://redpen.io/kf16f384972d804249

Same as on the old site, joining will:

• fork project in users github
• star project in users github
• if users 'introduce themselves' what they wrote should appear as a new ticket in the repo (not a required field)

User must be logged into site for this to happen. If they are not logged in then they see https://redpen.io/sm291d779b25ac292d