Feature/limit user role by email

[ivanlopez]

This feature allows for site admins to lock down what email domain a new user needs to have in order to be given a specific role. On a single site if you attempt to create a new user with a role and email that is not in line with the settings set the user will not be created and a notice informing you that you cant create that specific user with that role will be presented. On a multisite, the user would get generated but not added to the site. You would get a notice that the user could not be added to the site.

cc: @tlovett1

[ivanlopez]

@tlovett1 @helen copy updates have been made based on Melissa's recommendations.

[ivanlopez]

I have another update I need to push to this PR that provides cleaner notices when on a multi-site.