nicknames

[nicknames must only use NICK_C chars, must not start with a '-' or a digit](https://github.com/lp0/charybdis/blob/imaginarynet/ircd/match.c#L668)


limit is 30 characters, needs to handle conflicts

-----

potential solution:

```
uname = sanitise(uname[:30])
if clash(uname) {
    discriminator = getdiscriminator(u)
    uname = uname[:30-len(u) - 2] + "~d" + discriminator
}
```