Replace mass_comment.py with an automated solution

[damccorm]

Currently mass_comment.py helps with automatically running all post commits, but it relies on a duplicate list of PostCommit run phrases, which can get out of date (BEAM-13951). We should replace this with a solution that directly queries the Jenkins API for the list of PostCommits, then triggers them (either through the Jenkins API, or through comments on the PR).

Imported from Jira BEAM-14392. Original Jira may contain additional context.
Reported by: bhulette.