users: fix bug in changing usernames.

Fix the change username page not showing the user's current name when a mod is changing a user's name.

app/controllers/user_name_change_requests_controller.rb

@@ -6,7 +6,7 @@ class UserNameChangeRequestsController < ApplicationController
   skip_before_action :redirect_if_name_invalid?
 
   def new
-    user = User.find_by_name(params[:id]) || CurrentUser.user
+    user = params[:id].present? ? User.find(params[:id]) : CurrentUser.user
     @change_request = authorize UserNameChangeRequest.new(user: user, **permitted_attributes(UserNameChangeRequest))
     respond_with(@change_request)
   end

app/views/user_name_change_requests/new.html.erb

@@ -6,13 +6,13 @@
       <h1>Change Name</h1>
     <% end %>
 
-    <% if CurrentUser.user.name_invalid? %>
+    <% if @change_request.user.name_invalid? %>
       <p> Your current username is invalid. You must change your username to continue
       using <%= Danbooru.config.canonical_app_name %>.</p>
 
       <p>
-        Current name: <b><%= CurrentUser.user.name %></b>.<br>
-        Error: <%= CurrentUser.user.name_errors.full_messages.join(". ").html_safe %>.
+        Current name: <b><%= @change_request.user.name %></b>.<br>
+        Error: <%= @change_request.user.name_errors.full_messages.join(". ").html_safe %>.
       </p>
     <% end %>