Detect permissions over repos

[mihneadb]

Need

We don't want to make unnecessary requests.

Deliverables

• river detects at start time if the credentials have push rights on the given repo
• it uses this knowledge to fetch or not the Contributors data (it is restricted to push permissions)

Solution

• query https://developer.github.com/v3/repos/#get for the repo and look at the permissions dict
• store hasPush in a boolean
• check hasPush before fetching Collaborators