Add the ability to generate static images from graphs

continuing the discussion from https://github.com/influxdata/chronograf/issues/4712

as a user, I would like a way to statically embed a graph into a report, email, slack message, markdown, or any html page. I would like a url which would return a static image which could be used in an `` tag or ``.

Start by reading the linked Chronograf discussion in issue #4712 and the comments here to understand the proposed scope and prior decisions. Define the endpoint and rendering path needed to return a static graph image usable in an img tag, Markdown, email, Slack, or HTML, and verify the result against those embedding scenarios.

Written by the indexing model from the issue text.