Write Cloud Functions for reverse lookup of user emails

[aaga]

Sometimes we need to email specific users (e.g. if they signed up with an expectation of compensation).

We need to write a Firebase Cloud Function that takes a list of rally-ids and logs the corresponding list of email addresses. The reverse lookup can be done in the extensionUsers collection.

We must make sure that this function won't expose any information publicly.

[aaga]

This is currently blocked on Functions not deploying correctly:

#595