[BUG] Username length < 5 is not allowed, but user does not have a chance to know it

[picolino]

🐛 Bug Report

We have hardcoded username length min value:
https://github.com/skillaztech/Skillaz.SecureChat/blob/master/src/Chat.fs#L688

But UI does not have any restriction or validation logic for such case. So for end user it seems like username with 4 or less length are just not passes to other machines.

Expected behavior

When user enters username with length less than 5 symbols - we should restrict or validate that.

Reproduction steps

1. Launch SSC and connect to another peer
2. Change username on one machine to username with length < 5 symbols
3. No username changes on second machine will appear

Version: 1.0.0