Add auto_deploy command to display environment status

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690959ed - Update gitlab gem to 4.12.0

b588adbc - Add Slack.mrkdwn method

1e3dd112 - Store and expose host in Gitlab::Client

This renames DEFAULT_ENDPOINT to DEFAULT_HOST, since we were providing the /api/v4 endpoint for non-gitlab.com hosts.

Now we expose the resulting host setting through a reader, which will be used in a future command.

d04e31bc - Add version and commit_refs to Gitlab::Client

838c27ac - Add support for Block Kit to Slack::Message

50054a8b - Add auto_deploy command

The first iteration of this command will display the version and auto-deploy branch status for Production, Canary, and Staging.