Deploy boards

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.

| Success Response | Back off pooling | 204 response | With pods |--------|-------|---|---|---| | | | |

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

Closes #1589 (closed)