Searching for assignable user by email doesn't work

Issue Summary

It's no longer possible to search for assignable user by email in 8.19+.

Steps to Reproduce

1. Create a test user where email address is different than full name and username. Example:

   Full name	John Smith
   Username	jsmith
   Email address	ap86007@example.com

2. Open an existing issue and edit the assignee.
3. Start typing the email. 
4. Do the same with a new issue in the assignee field. 

Expected Results

John Smith should be highlighted as a match. This used to be the case before 8.19.1. 

Actual Results

John Smith is not matched.

Workaround

Search using full name or username.