repos: introduce low level slug check of repo and group names The high level web forms already slug-ify repo and repo group names. It might thus not create the exact repo that was created, but the name will be "safe". For API, we would rather have it fail than not doing exactly what was requested. Thus, always verify at low level that the provided name wouldn't be modified by slugification. This makes sure the API provide allow the same actual names as the web UI. This will only influence creation and renaming of repositories and repo groups. Existing repositories will continue working as before. This is a slight API change, but it makes the system more stable and can prevent some security issues - especially XSS attacks. This issue was found and reported by Kacper Szurek https://security.szurek.pl/