Set maximum username length to 18 for consistency
Set maximum username length to 18 for consistency

The Pokemon Showdown server already enforced a maximum username length
of 18 characters, but the login server allowed registering names up to
63 characters long.

submindraikou/Pokemon-Showdown-Client,QuiteQuiet/Pokemon-Showdown-Client,QuiteQuiet/Pokemon-Showdown-Client,panpawn/Gold-Client,Zarel/Pokemon-Showdown-Client,sirDonovan/Pokemon-Showdown-Client,panpawn/Gold-Client,sirDonovan/Pokemon-Showdown-Client,Zarel/Pokemon-Showdown-Client,sirDonovan/Pokemon-Showdown-Client,submindraikou/Pokemon-Showdown-Client,asgdf/Pokemon-Showdown-Client,Zarel/Pokemon-Showdown-Client,asgdf/Pokemon-Showdown-Client,Zarel/Pokemon-Showdown-Client,submindraikou/Pokemon-Showdown-Client,panpawn/Gold-Client,asgdf/Pokemon-Showdown-Client,asgdf/Pokemon-Showdown-Client,QuiteQuiet/Pokemon-Showdown-Client,QuiteQuiet/Pokemon-Showdown-Client,sirDonovan/Pokemon-Showdown-Client