Add the ability to search users by username

[samuel-yeom]

In the "Users" page for the staff, we want to be able to search for users by username.

[dlareau]

Added in 4ef2ef5. Some notes that may or may not apply to what you are doing:

• You basically never want to change the user object for a person
  • The only real scenario I can think of is if you needed to make a person puzzlehunt staff
  • Basically everything else you would want to change about a user should be editable by that user via https://puzzlehunt.club.cc.cmu.edu/user-profile/ which is accessible by clicking their username in the upper right corner when signed in.
  • If they forgot their password the login page has a link to reset it via email
  • Make sure to never change the email for a CMU login based account and make sure to always delete users/persons in pairs, leaving just one will result in undefined behavior
• If you want the Person object associated with a user the Person search bar searches all the user fields as well (first name, last name, email and username)