Remove session controller actions other than delete

and do some general cleanup around sessions controllers and the
sign_in_as test helper

test/controllers/sessions/launchpad_controller_test.rb

@@ -6,14 +6,16 @@ class Sessions::LaunchpadControllerTest < ActionDispatch::IntegrationTest
 
     assert_response :success
 
-    assert_equal cookies[:_fizzy_launchpad_sig], "test-sig"
+    assert_select "form input#sig" do |node|
+      assert_equal node.length, 1
+      assert_equal node.first["value"], "test-sig"
+    end
   end
 
   test "create establishes a session when the sig is valid" do
     user = users(:david)
-    cookies[:_fizzy_launchpad_sig] = user.signal_user.perishable_signature
 
-    put session_launchpad_path
+    put session_launchpad_path(params: { sig: user.signal_user.perishable_signature })
 
     assert_redirected_to root_url
     assert parsed_cookies.signed[:session_token]
@@ -21,9 +23,8 @@ class Sessions::LaunchpadControllerTest < ActionDispatch::IntegrationTest
 
   test "returns 401 when the sig is invalid" do
     user = users(:david)
-    cookies[:sig] = "not-valid"
 
-    put session_launchpad_path
+    put session_launchpad_path(params: { sig: "invalid" })
 
     assert_response :unauthorized
   end