Use transactions properly when building tasks from email

Summary: Fixes T5859. This doesn't change much, but makes the transaction record a little more accurate and activates stuff like `#hashtags` and `{F123}` causing policy associations.

Test Plan: Used `bin/mail receive-test` and mail receiver script to send bug mail, saw hashtags imply projects.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5859

Differential Revision: https://secure.phabricator.com/D10229