Deploy boards:

Not all environments have a deploy board. Only the ones with rollout_status_path key.
When the user clicks in the arrow we make a request to fetch deploy board data.
If the request is successful we render the deploy board
If the response is 204 we use backoff pooling and make at least 3 requests until we give up and show an error message.
If the request is unsuccessful we render a flash message in the top of the screen.

In this Merge Request:

Created a Vue component for deploy boards.
- This component is a smart component and it's responsible to make a request to the API
Created a Vue component for each instance
- This is a dumb component that renders provided data
Create an utility function for backoff pooling.
- This function was added as an utility because it will be needed in some other places. See this comment for more information: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1233#note_23693952
Added methods to the already existing code base to render deploy boards.
Added tests.
Used flexbox to guarantee a fully responsive layout for the instances.