Add organizations to Devise email routes v2

What does this MR do and why?

This reverts merge request !212005 (merged) and re-instates the changes in !209346 (merged)

Also:

• Adds end-to-end specs to verify that the links sent in Devise emails work as intended
• Fixes a minor bug in confirmations flow uncovered by the new specs
• Adds a feature flag devise_email_organization_routes to use the new routes in sent emails, so we can roll back quickly if any issues are discovered

References

• Original issue: #562035 (and related unique index de-scoping issues)
• !209346 (merged)
• !212005 (merged)
• New feature flag issue for this MR: #580586

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