Use button_to to generate a POST form, disable turbo with OmniAuth

This changes the OmniAuth "sign in" links to use buttons, which can be
wrapped in an actual HTML form with a method POST, making them work
better with and without Turbo in the app. It doesn't require rails/ujs
anymore in case of a non-Turbo app, as it previously did with links +
method=POST.

Turbo is disabled for those OmniAuth buttons, as they simply don't work
trying to follow the redirect to the OmniAuth provider via fetch,
causing CORS issues/errors.

test/integration/omniauthable_test.rb

@@ -128,13 +128,17 @@ class OmniauthableIntegrationTest < Devise::IntegrationTest
 
   test "generates a link to authenticate with provider" do
     visit "/users/sign_in"
-    assert_select "a[href=?][data-method='post']", "/users/auth/facebook", text: "Sign in with FaceBook"
+    assert_select "form[action=?][method=post]", "/users/auth/facebook" do
+      assert_select "input[type=submit][value=?]", "Sign in with FaceBook"
+    end
   end
 
   test "generates a proper link when SCRIPT_NAME is set" do
     header 'SCRIPT_NAME', '/q'
     visit "/users/sign_in"
-    assert_select "a[href=?][data-method='post']", "/q/users/auth/facebook", text: "Sign in with FaceBook"
+    assert_select "form[action=?][method=post]", "/q/users/auth/facebook" do
+      assert_select "input[type=submit][value=?]", "Sign in with FaceBook"
+    end
   end
 
   test "handles callback error parameter according to the specification" do

test/integration/timeoutable_test.rb

@@ -109,7 +109,7 @@ class SessionTimeoutTest < Devise::IntegrationTest
     follow_redirect!
 
     assert_response :success
-    assert_contain 'Sign in'
+    assert_contain 'Log in'
     refute warden.authenticated?(:user)
   end