Make getting a user by the username case insensitive

What does this MR do?

This merge request makes sure that when creating a new user object searching by username, it does so in a case sensitive manor.

What are the relevant issue numbers?

Closes #38304 (closed)
Closes #27229 (closed)
Possibly solves #47601 (moved)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated (After searching, nothing in the docs suggest usernames are sensitive or insensitive),
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

lib/api/users.rb

@@ -153,7 +153,6 @@ def reorder_users(users)
         requires :username, type: String, desc: 'The username of the user'
         use :optional_attributes
       end
-      # rubocop: disable CodeReuse/ActiveRecord
       post do
         authenticated_as_admin!
 
@@ -164,17 +163,16 @@ def reorder_users(users)
           present user, with: Entities::UserPublic, current_user: current_user
         else
           conflict!('Email has already been taken') if User
-              .where(email: user.email)
+              .iwhere(email: user.email)
               .count > 0
 
           conflict!('Username has already been taken') if User
-              .where(username: user.username)
+              .iwhere(username: user.username)
               .count > 0
 
           render_validation_error!(user)
         end
       end
-      # rubocop: enable CodeReuse/ActiveRecord
 
       desc 'Update a user. Available only for admins.' do
         success Entities::UserPublic
@@ -196,11 +194,11 @@ def reorder_users(users)
         not_found!('User') unless user
 
         conflict!('Email has already been taken') if params[:email] &&
-            User.where(email: params[:email])
+            User.iwhere(email: params[:email])
                 .where.not(id: user.id).count > 0
 
         conflict!('Username has already been taken') if params[:username] &&
-            User.where(username: params[:username])
+            User.iwhere(username: params[:username])
                 .where.not(id: user.id).count > 0
 
         user_params = declared_params(include_missing: false)