build, upload, and host graphviz output

To avoid more manual effort, VetBot should build graphviz visualizations of suspicious callgraphs, upload them somewhere (probably Google's version of an S3 Bucket) and embed the picture into the uploaded file in the issue.

Originally I wanted to keep the images in the same repository as the issues, but I don't like that so much since it uses at least one more API request (and, anecdotally, we're already hitting the rate limit between VetBot and TrackBot).

This is a nice-to-have feature, which should probably be done anyway to make the UX better for our crowdsourcers.

No files, tests, or entry points are identified in the issue. Start by locating VetBot's suspicious-callgraph generation and issue-upload flow, then define the hosting and embedding path; done means generated Graphviz images are uploaded and embedded without the extra repository/API overhead described.

Written by the indexing model from the issue text.