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