Actually validate admin username/password in Windows installer

## 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.

Locate the Windows installer project and review how the admin username and password prompts currently validate input. Read issues #2263 and #2188 and evaluate the proposed StringTools WiX extension; done means unsupported characters block progression while valid credentials continue.

Written by the indexing model from the issue text.