R1:fd66ab55791e

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…