Allow changing email address within domain

[jace]

Hasjob doesn't allow changing an email address once a job is listed, since (a) the domain is crucial to tagging the post and (b) we want to confirm the email address works.

However, changing the email is a common-enough request, so we should allow changes within the same domain. Requirements:

1. UI: Make it clear the domain can't be changed, perhaps with an AnnotatedTextField and the current domain as a suffix.
2. Change the form so that poster_email is an AnnotatedTextField instead of EmailField in this scenario. Maybe by having two fields and selectively rendering (since we already have selective rendering code in place).
3. Since we need to reverify the email but can't store the new address in the email field until it is verified, we send a cryptographically signed URL that includes the new email (perhaps just the username part to avoid putting an email address in the URL). This goes to a different endpoint than /confirm, maybe a /reconfirm.
4. When the new address is verified, we change it.

(Finding ways to remove email verification is a separate discussion.)