Admin users page: filter by username

Allow admins to filter the user list/index by username (partial matches
allowed)

spec/features/admin/users_spec.rb

@@ -15,6 +15,8 @@ RSpec.describe "Admin: User management", type: :feature do
       .and_return({ uid: user.cn, mail: user.email, display_name: "Freddy" })
     allow_any_instance_of(LdapManager::FetchAvatar).to receive(:call)
       .and_return(nil)
+    allow_any_instance_of(LdapService).to receive(:search_users)
+      .and_return([])
 
     login_as admin, :scope => :user
   end
@@ -43,6 +45,30 @@ RSpec.describe "Admin: User management", type: :feature do
     expect(user.invitations.count).to eq(5)
   end
 
+  describe "User index page" do
+    it "lists all users" do
+      visit admin_users_path
+
+      expect(page).to have_link "jimmy"
+      expect(page).to have_link "alfred"
+    end
+
+    scenario 'Filter users by username' do
+      visit admin_users_path
+
+      fill_in "username", with: "alf"
+      click_button "Filter"
+
+      expect(page).to have_link "alfred"
+      expect(page).to have_no_link "jimmy"
+
+      click_link "Remove filter"
+
+      expect(page).to have_link "jimmy"
+      expect(page).to have_link "alfred"
+    end
+  end
+
   scenario 'Remove invitations from account' do
     3.times { Invitation.create(user: user) }
     expect(user.invitations.count).to eq(3)