FIX: lowercase username for add/rem group members

This fix searches for users based on the downcased username so that if
you pass in usernames to add/remove from a group and you don't have the
casing just right it will still find the correct users.

I updated the tests to add a username that has a mix of upper and
lowercase letters to verify this functionality.

app/controllers/groups_controller.rb

@@ -484,7 +484,7 @@ class GroupsController < ApplicationController
 
   def users_from_params
     if params[:usernames].present?
-      users = User.where(username: params[:usernames].split(","))
+      users = User.where(username_lower: params[:usernames].split(",").map(&:downcase))
       raise Discourse::InvalidParameters.new(:usernames) if users.blank?
     elsif params[:user_ids].present?
       users = User.where(id: params[:user_ids].split(","))

spec/requests/groups_controller_spec.rb

@@ -856,7 +856,7 @@ describe GroupsController do
 
       context "is able to add several members to a group" do
         let(:user1) { Fabricate(:user) }
-        let(:user2) { Fabricate(:user) }
+        let(:user2) { Fabricate(:user, username: "UsEr2") }
 
         it "adds by username" do
           expect do
@@ -1069,7 +1069,7 @@ describe GroupsController do
       context '#remove_members' do
         context "is able to remove several members from a group" do
           let(:user1) { Fabricate(:user) }
-          let(:user2) { Fabricate(:user) }
+          let(:user2) { Fabricate(:user, username: "UsEr2") }
           let(:group1) { Fabricate(:group, users: [user1, user2]) }
 
           it "removes by username" do