User browser filter needs case insensitive search and more options

Email addresses are case insensitive, therefore the user browser filter should perform a case insensitive search on email addresses. Most people use lower case characters in their email address, but some use upper case letters too, and currently they are very difficult to find.

In addition, "filter by username" and "filter by fullname" fields would be very useful. Many users have strange and not intuitive email addresses. Our administrator knows persons by their full name, not necessarily by their email address.