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

app/controllers/snippets_controller.rb

@@ -24,10 +24,9 @@ class SnippetsController < ApplicationController
   layout 'snippets'
   respond_to :html
 
-  # rubocop: disable CodeReuse/ActiveRecord
   def index
     if params[:username].present?
-      @user = User.find_by(username: params[:username])
+      @user = User.find_by_username(params[:username])
 
       return render_404 unless @user
 
@@ -39,7 +38,6 @@ def index
       redirect_to(current_user ? dashboard_snippets_path : explore_snippets_path)
     end
   end
-  # rubocop: enable CodeReuse/ActiveRecord
 
   def new
     @snippet = PersonalSnippet.new