Validate email addresses upon registration

Nobody has claimed this yet.

# TL;DR
Validate email addresses upon registration to ensure we can actually reach participants and that researchers are associated with their stated email address.

# Narrative
As a new user, I may make a typo in my email address but not realize that email with compensation will not reach me.

As a researcher, I want assurance that collaborators listed with a particular email address are actually the person they say they are.

# Acceptance Criteria
A precise description of how/when this issue will be considered closed, e.g.:

- [ ] Registration workflow includes email to address to activate account
- [ ] Email address change happens upon confirming access to new account

Start by tracing the registration workflow and the existing account email-change handling. Define where activation confirmation is required and how access to a new address is confirmed; done means registration activation and confirmed email changes satisfy both listed acceptance criteria.

Written by the indexing model from the issue text.