Spiff github app to show before and after visuals on PRs

[burnettk]

SpiffWorkflow is commonly used with process models in a git repo. When this process model repo is updated, reviewers of the PR could benefit from seeing what changed visually, since BPMN is a visual notation, and since reading XML diffs is not all that it's cracked up to be.

Perhaps the app would receive API requests only for PRs, and if it hadn't yet commented on the PR, it could take a look at the changed files and determine if any .bpmn files had changed. Perhaps if fewer than n files changed (could start with only supporting one file, so PRs with many changed files would be ignored), it could generate an image from the "before" state of the bpmn and the "after" state. if those images differed, it could add a comment to the PR including the before and after visuals for the benefit of the reviewers (and maybe even the author, to confirm they are changing what they think they are changing). There is a nodejs tool based on bpmn-js, https://github.com/bpmn-io/bpmn-to-image, that can apparently render images from .bpmn files, and there are probably others, but the language of the app could be whatever we wanted, as it could be decoupled from the other parts of the codebase.