Prefer importing ka_secrets to secrets.
Prefer importing ka_secrets to secrets.

Summary:
This will only work when runniing alertlib inside the webapp repo, but
in that case it will auto-update an out-of-date secrets file rather
than just-dying like it does today.

Test Plan:
python -c 'import alertlib'
succeeds without error, as does
env PYTHONPATH=$HOME/khan/webapp python -c 'import alertlib'

(though I have to fix up ka_secrets.py first to export the new secrets.)

Reviewers: alpert

Differential Revision: http://phabricator.khanacademy.org/D10175

Khan/alertlib,mroth/alertlib