Username policies

[AgostiniAlessandro]

Currently password policies can be customised with various options (eg minimum length, the presence of at least one special character,...), we want to replicate some of these features for the username, namely adding a minimum and maximum length and forbidding the use of whitespaces and special characters.

For reference this is where the username gets validated:

Orchard/src/Orchard.Web/Modules/Orchard.Users/Services/AccountValidationService.cs

Line 39 in 028e2e4

public bool ValidateUserName(AccountValidationContext context) {