Add labels to issues

[redrabbit]

We want to associate user-defined labels to issues. Similar to GitHub.

• [x] GitGud.IssueLabel schema has name, description and color fields.
• [x] Each repository has a set of default labels created in GitGud.Repo.create/1.
• [x] Maintainers with :admin rights can create/edit/delete labels.
• [x] Issues can have labels associated (many-to-many).
• [x] GraphQL schema provides a new updateIssueLabels(). Receiving labels ids to push/pull.
• [x] An issue event containing pushed/pulled labels is emitted on each updateIssueLabels() call.