Arcyd: allow multiple repo admin emails

Allow multiple repo admin email addresses and add support for them to be
specified on any combination of:

- the phabricator instance config
- the repohost config
- the repo config

Raise if a repository has no admin email addresses during setup and
arcyd startup.

Change the name of the admin email argument to '--admin-emails', a quirk
of argparse means that the '--admin-email' argument is still supported.

Adjust test_shell.sh to use multiple emails from multiple sources.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/loop_test.sh
$ ./testbed/arcyd/acceptance_tests.sh

try different combinations of emails in
$ ./testbed/arcyd/test_shell.sh