chore: replace managePrimaryEmail express api by next action

[LucasCharrier]

Réimplemente les routes express en next server action :

• admin/mattermost
• managePrimaryEmail
• manageSecondaryEmail

[gitguardian]

⚠️ GitGuardian has uncovered 1 secret following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

🔎 Detected hardcoded secret in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
967700	Triggered	Username Password	17d1b1e	tests/test-user.ts	View secret

🛠 Guidelines to remediate hardcoded secrets

1. Revoke and rotate the secret.

2. If possible, rewrite git history with git commit --amend and git push --force.

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[revolunet]

possible de faire une 1ere PR avec juste les changes mattermost ?