Allow bugs@ addresses to blanket-accept tasks
Summary: Allow configuration of a default author for bugs@ emails which don't
correspond to a known system user.
Test Plan: Configured a default author, sent some mails from nonsense addresses,
tasks were created.
Reviewers: davidreuss, jungejason, nh, tuomaspelkonen, aran
Reviewed By: aran
CC: aran, epriestley, ide
Differential Revision: 1013