Extract Users::JoinsController from overloaded UsersController

Good code smell is when the before_action callbacks stack up but can't
be shared across actions

test/controllers/users/joins_controller_test.rb

@@ -0,0 +1,32 @@
+require "test_helper"
+
+class Users::JoinsControllerTest < ActionDispatch::IntegrationTest
+  test "new" do
+    identity = Identity.create!(email_address: "new.user@example.com")
+    identity.memberships.create(tenant: ApplicationRecord.current_tenant, join_code: Account::JoinCode.sole.code)
+    sign_in_as identity
+
+    get new_users_join_path
+    assert_response :ok
+  end
+
+  test "new with invalid params" do
+    identity = Identity.create!(email_address: "new.user@example.com")
+    membership = identity.memberships.create(tenant: ApplicationRecord.current_tenant, join_code: "PHONY")
+    sign_in_as identity
+
+    get new_users_join_path
+    assert_redirected_to unlink_membership_url(script_name: nil, membership_id: membership.signed_id(purpose: :unlinking))
+  end
+
+  test "create" do
+    identity = Identity.create!(email_address: "newart.userbaum@example.com")
+    identity.memberships.create(tenant: ApplicationRecord.current_tenant, join_code: Account::JoinCode.sole.code)
+    sign_in_as identity
+
+    assert_difference -> { User.count }, +1 do
+      post users_joins_path, params: { user: { name: "Newart Userbaum" } }
+      assert_redirected_to root_path
+    end
+  end
+end