Preselect existing usernames from dropdown during mapping

[roborourke]

Needs some thought re indicating taken usernames but auto-sets the dropdown if a user is found then checks if they belong to the current site on multisite

[JJJ]

This seems OK, but maybe would benefit from big-network checks:

• get_blogs_of_user() can be slow on large installs
• Maybe repeated calls to get_user_by() can be lumped into 1 get_users() call?

[roborourke]

@JJJ thanks for the review and good advice. I'll follow the approaches used in the importer to make this more efficient

[rmccue]

@roborourke Still want to update this one per @JJJ's comments?

[roborourke]

@rmccue yeah I think I should but I'm stretched thin right now. Will try and make some time next week