[FIX] Reg: Async username validation

RT-2351 #inreview
When more than two characters are entered into the signup username
field, the async availability check is started. Entering invalid
characters while the network request is pending causes the false
'Available' message to appear.
Cancel the async request once any additional characters are entered.
Merge pull request #1600 from jublonet/feature/2351-reg-username-validation-check

[FIX] Reg: Async username validation