Always use the same set of transactions to generate mail and mail tags

Summary:
See PHI307. Currently, when reviews undraft, we retroactively add in older activity to the mail ("alice created this revision...").

However, we don't add that activity to the mail tags, so the relevant tags (like "revision created") are dropped forever.

Instead, use the same set of transactions for both mail body and mail tag construction.

This should be obsoleted in the relatively near future by T10448, but it's a better/more correct behavior in general and we probably can't get rid of tags completely for a while.

Test Plan:
Applied patch, created a revision with builds, saw it auto-undraft after builds finished. Used bin/mail list-outbound and bin/mail show-outbound to see the mail. Verified that it included retroactive text ("created this revision") AND retroactive tags.

Note that the tag for "A new revision is created" is DifferentialTransaction::MAILTAG_REVIEW_REQUEST with literal value differential-review-request.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18941