Add a front route for user reconciliations

[Ash-Crow]

Feature Request

Is your feature request related to a problem or unsupported use case? Please describe.
Required for #1708 : when a UserReconciliation is created, two confirmation emails are sent per email pair (active and inactive), each with a link to validate that the user controls the address.

Describe the solution you'd like
When a user clicks a link with the following format: https://<root_url>/user_reconciliations/<user_type>/<confirmation_id>, the equivalent backend URL in the API is called (https://<root_url>/api/<api_version>/user_reconciliations/<user_type>/<confirmation_id>), which processes the request in the background.

The user is then redirected to a simple confirmation page telling if everything is alright.

The value of <user_type> can be either active or inactive.
The value of <confirmation_id> is an UUID.

Discovery, Documentation, Adoption, Migration Strategy
The confirmation email:

[Ash-Crow]

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