Allow admin-usernames with more than 50 characters

Already analyzed: https://itrac.eur.ad.sag/browse/IOT-5966

---

When creating a tenant the "ADMINISTRATOR'S USERNAME" has to have <= 50 characters. 

This also counts for direct API call (property "adminName"). 

Idea is to remove this legacy validation to allow admin-usernames with more than 50 characters, as usually email-address equals the username.

Siemens issue:
Siemens has the issue that admin e-mail-address has to be equal to username. 
In case of e-mail-addresses larger than 50 characters this is not possible. 

- For normal users the limit is 254 instead of 50 characters 
- Minor: UI validation actually allows 254 characters, only backend validates to 50 characters length 

Dev tools shows 422 Unprocessable Entity for payload.. 
{sendPasswordResetEmail: false, storageLimitPerDevice: 0, company: "morethan50",…} 
adminEmail: "iamanemailaddress.morethan50chars@somebigcompanyname.com" 
adminName: "iamanemailaddress.morethan50chars@somebigcompanyname.com" 
adminPass: "Test1234#" 
company: "morethan50" 
contactName: "r" 
contactPhone: "" 
domain: "morethan50.cumulocity.com" 
sendPasswordResetEmail: false 
storageLimitPerDevice: 0