wip: users updating username or email are forced to pick a unique set

Add UserCommands::checkUniqueIdentity utility to check if user is updating to a valid username/email
SiteAdmin user management form updated to use this
unique username/email - part 2

Update Site Admin user management form to check for unique username/email
Fix check_unique_identity API call when creating a new user
Modified how UserCommands::checkUniqueIdentity sets usernameMatchesAccounta and emailMatchesAccount
Update User Management invite system to add a user if email address already exists
Merge remote-tracking branch 'lf/feature/uniqueEmail'