Validate new user username

Show a descriptive message when the username input doesn't match a modified version of the regex used by the backend

src/apps/dashboard/routes/users/add.tsx

@@ -205,6 +205,7 @@ const UserNew = () => {
                             type='text'
                             id='txtUsername'
                             label='LabelName'
+                            validator={{ pattern: '^([\\w \\-\'._@+]*)([\\w\\-\'._@+])([\\w \\-\'._@+]*)$', errMessage: 'Username must not be empty and contain only numbers, letters, spaces, or the following symbols -\'._@+' }}
                             options={'required'}
                         />
                     </div>