firehose should add some tools to make sure people are publishing from the intended commit #308

Currently, firehose publishing generally publishes from the head commit. This generally happens thuswise:

• firehose writes a publishing comment into a PR
• that PR eventually lands
• a contributor with commit access clicks on the 'publish' link in the PR; that opens a pre-populated release page that (when created) will trigger publishing

If there are many packages in a repo, we don't always want to publish from the latest commit; other commits may have landed after the original PR. When creating the release page info, we should add some text to encourage the user to tag a specific commit (which will frequently, but not always, be head). The commit to tag add can be selected from the 'target', 'recent commits' dropdown: