allow user to change their Zupass email

[artwyman]

They should be able to keep their tickets, without relying on reissuance.
They should be able to keep their Semapore ID.

This could be a subset of a broader feature to support multiple emails per account, but may need to happen sooner because we have no other answer to changing a user's email without ticket reissuance.

[robknight]

Doing this would probably cause the user's email PCD to be deleted and replaced via the feed.

However, they may already have taken a backup of it, and so could end up with multiple email PCDs, which could cause confusion.