Add search type: Username

Purpose: Make it possible to better narrow search to find user records

User story statement(s):

As a librarian who is trying to locate a user record in the Users app
I want to be able to search the username field specifically
so that I can find a specific user more quickly than using the Keyword search which will produce more "noise" in the search results

Scenarios:

1. Scenario:
   • Given the search and filter pane for Users
   • When displayed
   • Then a "search type" menu should appear above the search box as shown in the linked mockup
2. Scenario:
   • Given the search type menu
   • When opened
   • Then a "Username" option should be available (options displayed alpha under the default)

Wireframe: https://drive.google.com/file/d/1v9BAMunYLCF8VAJB8IY45btEEdnTqYiR/view?usp=sharing

Split to UIU-1903, which deals with explicit truncation.