Note on export/import of users and modified patch

[Trevor McKay]

FYI, just a note on the modified patch in my previous email.

If you have a modified cumin DB with an 'external' column in the Users
table and you want to save the user data, you should be able to use
cumin-admin export-users to save the user database.

Then apply the modified patch and reschema (or remove the external
column from the User table in Postgres by hand if there is other stuff
in the DB you want to keep).  Now the User table will match the original
schema.

Next, cumin-admin import-users <usersfile>.  The updated cumin-admin
should correctly add the external users back in using the zero-length
password method.

Best,

Trevor