Changes handling collaborators

[mpsonntag]

Currently collaborators are added with repository permission level can-push which cannot be changed.

• Add PUT route /users/{user}/repos/{repo}/collaborators/{username}/permission to update the repository permission level of a collaborator. The user facilitating this change has to at least have the is-admin repo access level for the repository in question. Only valid store.repo.AccessLevel are supported.
• Edit listRepoCollaborators to include the repository permission level of a collaborator to the response JSON.

[mpsonntag]

putRepoCollaborator basically already does exactly what is required to update the repository permission level of a collaborator. Instead of creating yet another route it would be best to just use this for both creating a new collaborator and updating the access level of an existing one.