DNN-6093 Changes for streamlining "Use Email Address as Username" 

This is certainly nothing that can be merged right away. It does improve a lot on the end user experience though.

The main issue IMHO before this can be merged are upgrade scenarios. With the changes made here the site setting "Use Email Address as Username" is depending on "RequireUniqueEmail" in web.config set to true and also require that no duplicate emails exist in the database.

In installations where the setting is turned off there shouldn't be any issues in upgrade scenarios since the code behind in site settings is now validating those requirements before turning on the feature.

The question is what to do in upgrades where Use "Email Address as Username" is turned on already. Obviosly we cannot swipe out duplicate email addresses automatically. This work needs to be done by the site admin manually.

The document attached in https://dnntracker.atlassian.net/browse/DNN-6093 does outline the changes made in more detail btw.