Users: add fields to filter users by username or email

[DonWillems]

As an admin I want to easily find a user, so that I don't have to waste time clicking through the paginated list.

• Add filter field left of the '+ add user' button in both panels
• Filter the users shown below based on the match in username or email address
• Match is case insensitive, and can be at the start/middle/end of the username or email
• Decide whether filtering on front or backend is best
• Filter on input change, add a debounce of 250ms