Allow email subject prefixes to be configured

Summary:
This is just fluff to let me mailfilter my local sandbox. Would also allow the
Facebook install to return to "[diff]" if eletuchy is still unhappy about this
change.

Test Plan:
Triggered maniphest/differential emails, had normal prefixes. Overrode prefixes
in my custom config, got sandbox-unique prefixes.

Reviewed By: aran
Reviewers: jungejason, tuomaspelkonen, aran
CC: elgenie, aran
Differential Revision: 291