Challenge workflows - Stage 4

[krallin]

This updates the Dashboard to use the new email verification workflows to:

• Re-send email verifications (for new users).
• Create an email verification challenge (when updating the user's email).

For context, see: https://github.com/aptible/auth.aptible.com/pull/255

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Note: this includes https://github.com/aptible/dashboard.aptible.com/pull/777, but that PR is expected to ship earlier (see the rollout plan in https://github.com/aptible/auth.aptible.com/pull/255). The relevant commit is the other one (Stage 4).

cc @sandersonet @fancyremarker @blakepettersson

[krallin]

Moved to https://github.com/aptible/dashboard-private/pull/21