IntegrityError participants_email_address_key

[chadwhitacre]

This may or may not be a regression from or related to #4305, but marking as such for now because @jdorfman hit this post-#4305.

https://sentry.io/gratipay/gratipay-com/issues/280455978/

I'm almost positive we've seen this before in some fashion, though.

IntegrityError: duplicate key value violates unique constraint "participants_email_address_key"
DETAIL:  Key (email_address)=([email protected]) already exists.

  File "site-packages/algorithm.py", line 288, in run
    new_state = function(**deps.as_kwargs)
  File "aspen/algorithms/website.py", line 115, in get_response_for_resource
    return {'response': resource.respond(state)}
  File "aspen/http/resource.py", line 59, in respond
    content_type, body = super(Dynamic, self).respond(accept, state)
  File "aspen/simplates/__init__.py", line 169, in respond
    exec(self.pages[1], context)
  File "/app/www/~/%username/emails/modify.json.spt", line 49, in <module>
    participant.set_primary_email(address)
  File "gratipay/models/participant/email.py", line 225, in set_primary_email
    self._set_primary_email(email, cursor)
  File "gratipay/models/participant/email.py", line 240, in _set_primary_email
    """, dict(email=email, username=self.username))
  File "postgres/cursors.py", line 92, in run
    self.execute(sql, parameters)
  File "psycopg2/extras.py", line 288, in execute
    return super(NamedTupleCursor, self).execute(query, vars)