Allow todos to be marked as done

As a GitLab user, I want to be able to not only view but also check of my todos with GitDock.

🚧 Challenges

GitDock currently only requires access tokens with the read_api scope. However, this feature would need api to mark todos as done.

See !10 (closed) for my previous attempt.

We could solve this by allowing users to optionally provide an api access token (or re-authenticate with OAuth2 with the api scope). Ideally, GitLab would have more gradual scopes (like GitHub) but maybe gitlab-org/gitlab#329172 (closed) will add that. 👀

🖼 Design

Current UI	Suggestion