Make "these changes are now available in..." message appear to come from merged user when possible

[gwynne]

When the "release bot" functionality of Penny is triggered by an appropriately labeled PR, one of the actions it takes is to comment on the PR with "These changes are now available in [](link to new release)." This comment presently comes from the bot itself, and looks like this:

However, if the user who merged the PR (almost always a code owner) has linked their GitHub account to Penny via the OAuth flow, Penny has the ability to act "on behalf of" that user. In that case, Penny should use that ability to comment "as" the merging user (see the docs for auth-ing as a user for an example of what this would look like).

[gwynne]

Oh, and doing this would also avoid Penny always being listed as a contributor on PRs in GitHub's social previews when all ti did was add one comment.