Fix denied login for users with many repos.
Fix denied login for users with many repos.

`isCollaborator` was created in #491 to block login if a user did not have write (push) permissions to a repo, by going through the list of a users repos until it found the right one. It did not institute pagination, however, so if a user had enough repos that the one in question was on another page, the CMS would assume that they did not have permission and block the login.

This commit fixes the problem by calling the API for the specific repo instead of getting the whole list.

netlify/netlify-cms,Aloomaio/netlify-cms,netlify/netlify-cms,dopry/netlify-cms,netlify/netlify-cms,Aloomaio/netlify-cms,netlify/netlify-cms,dopry/netlify-cms