Changes handling collaborators

[Michael Sonntag]

Depends on issue G-Node/gin-repo#69.

Change RepoSettings.vue to support the following changes:

• Deal with the new collaborator JSON containing the repository permission levels when displaying existing collaborators.
• Change the default repository permission level from can-push to can-pull when adding a new collaborator to a repository.
• Display the repository permission level that a collaborator currently has (should currently be either can-pull or can-push).
• Add the option to either add or remove can-push to or from a collaborator.