Add sender option to alertlib emails

Summary:
This adds a sender argument which makes the sender 'alertlib <no-reply+sender@khanacademy.org>' for alertlib emails.

Assuming this looks ok, I'll clean up the pipeline emails to only use alertlib.

Test Plan:
Run locally and check logs. In devshell:
```
test@.. [12]: %paste
from third_party import alertlib
alertlib._TEST_MODE = False
a = alertlib.Alert('message body', summary='subject')
a.send_to_email('tony+test', sender='some-bot')
## -- End pasted text --
localhost:8080 [12]: <third_party.alertlib.Alert at 0x105fd8250>

test@.. [13]: a.send_to_email('tony+test') # without sender
localhost:8080 [13]: <third_party.alertlib.Alert at 0x105fd8250>
```

Logs:
{F16842}

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: chris, alan, jace

Differential Revision: http://phabricator.khanacademy.org/D8734