Actually validate admin username/password in Windows installer

[wohali]

Summary

The new CouchDB 3.0 Windows installer now prompts for an admin username and password at install time. #2263 and #2188 mean CouchDB won't handle usernames or passwords with certain special characters in them. The installer doesn't prevent this.

Desired Behaviour

Actually detect that this occurs during installer prompt time and don't allow to progress if bad chars are found in username or password.

Possible Solution

Use the StringTools WiX extension from https://github.com/dblock/msiext (EPL 1.0) to check. It isn't hard but I ran out of time before the 3.0 installer had to ship.

Additional context

It'd prevent surprises in a pretty small number of cases.