Allow searching for user accounts with specific substring in email address

[aklapper]

In Hatstall, I want to search for accounts with an email address of a specific domain (@example.com) which do not have Example.com as their enrollment, to fix their enrollment. Or for account names with Example in their name as that is a common way in our project to express that you work for Example.

(Reported this under sortinghat and not hatstall as requested by sduenas in internal support ticket 44)