no way to automatically import revoked CLAs

[Steven R. Loomis]

When making a minor update to the CLA, I have to re-import the signatories.

However, the import process doesn't have a way to import the 'revoked' field (see also #940).

To workaround this, I manually removed all rows with a 'revoked_at' column, and uploaded.

Organization is: https://github.com/unicode-org

Please add a way to import the 'revoked' field as well.