Add support for real-time review discussions

[redrabbit]

We have GitGud.CommitReview and GitGud.CommitLineReview providing support for comments on commits and commit diff lines.

It would be great if we could push real time events aka. subscriptions to the respective <CommitReview /> and <CommitLineReview /> React components.

Currently, we rely on Relay and Absinthe for GraphQL subscriptions. We might use Phoenix.LiveView in future (see #47).

[redrabbit]

Our GraphQL schema supports following subscriptions:

• commitLineReviewCreate/3 Subscribe to new line commit reviews.
• commitLineReviewCommentCreate/5 Subscribe to new line commit review comments.
• commitReviewCommentCreate/2 Subscribe to new commit review comments.
• commentUpdate/1 Subscribe to comment updates.
• commentDelete/1 Subscribe to comment deletions.

In order to receive new commit line review events, subscribe to commitLineReviewCreate/3.

Each commit line review and commit review subscribe to new events using their respective subscribe function (commitLineReviewCommentCreate/5 or commitReviewCommentCreate/2).

Each comment is responsible for subscribing to update and delete events.