[Enhancement #482] Support sending invitation links to new users

[lukaskabc]

Resolves #482

[gitguardian]

⚠️ GitGuardian has uncovered 7 secrets 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.

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Skipping this check run and merging your pull request will create secret incidents on your GitGuardian dashboard.

🔎 Detected hardcoded secrets in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
-	-	Generic Password	291e378	src/i18n/cs.ts	View secret
-	-	Generic Password	8ca5a30	src/i18n/cs.ts	View secret
-	-	Generic Password	8ca5a30	src/i18n/cs.ts	View secret
-	-	Generic Password	291e378	src/i18n/cs.ts	View secret
-	-	Generic Password	8ca5a30	src/i18n/cs.ts	View secret
-	-	Generic Password	291e378	src/i18n/cs.ts	View secret
-	-	Generic Password	8ca5a30	src/i18n/cs.ts	View secret

🛠 Guidelines to remediate hardcoded secrets

1. Understand the implications of revoking this secret by investigating where it is used in your code.
2. Replace and store your secrets safely. Learn here the best practices.
3. Revoke and rotate these secrets.
4. If possible, rewrite git history. Rewriting git history is not a trivial act. You might completely break other contributing developers' workflow and you risk accidentally deleting legitimate data.

To avoid such incidents in the future consider

• following these best practices for managing and storing secrets including API keys and other credentials
• install secret detection on pre-commit to catch secret before it leaves your machine and ease remediation.

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^{🦉 GitGuardian detects secrets in your source code to help developers and security teams secure the modern development process. You are seeing this because you or someone else with access to this repository has authorized GitGuardian to scan your pull request.}

[ledsoft]

See the inline comments. Mostly minor stuff, the main issue I have is the change of the HTTP method for creating an account.

[ledsoft]

One more round of comments after actually testing the functionality.