refactor: clean up git migration forms (#39672)

Only extracted shared code, removed unnecessary code, and use
form-fetch-action to replace RenderWithErrDeprecated, slightly
refactored JS to use data-global-init

Nothing else is changed, although there are still many problems.

Written by AI

Diff with hiding spaces:
https://github.com/go-gitea/gitea/pull/39672/changes?w=1

tests/integration/migrate_test.go

@@ -94,9 +94,9 @@ func TestMigrateGiteaForm(t *testing.T) {
 			"description": "",
 			"uid":         strconv.FormatInt(repoOwner.ID, 10),
 		})
-		resp = session.MakeRequest(t, req, http.StatusSeeOther)
+		resp = session.MakeRequest(t, req, http.StatusOK)
 		// Step 5: a redirection displays the migrated repository
-		loc := resp.Header().Get("Location")
+		loc := *test.ParseJSONRedirect(resp.Body.Bytes()).Redirect
 		assert.Equal(t, fmt.Sprintf("/%s/%s", ownerName, migratedRepoName), loc)
 		// Step 6: check the repo was created
 		unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{Name: migratedRepoName})

tests/integration/repo_migrate_test.go

@@ -10,6 +10,7 @@ import (
 	"testing"
 
 	"gitea.dev/modules/structs"
+	"gitea.dev/modules/test"
 	"gitea.dev/tests"
 
 	"github.com/stretchr/testify/assert"
@@ -23,7 +24,7 @@ func testRepoMigrate(t testing.TB, session *TestSession, cloneAddr, repoName str
 	link, exists := htmlDoc.doc.Find("form.ui.form").Attr("action")
 	assert.True(t, exists, "The template has changed")
 
-	uid, exists := htmlDoc.doc.Find("#uid").Attr("value")
+	uid, exists := htmlDoc.doc.Find(`input[name="uid"]`).Attr("value")
 	assert.True(t, exists, "The template has changed")
 
 	req = NewRequestWithValues(t, "POST", link, map[string]string{
@@ -32,7 +33,7 @@ func testRepoMigrate(t testing.TB, session *TestSession, cloneAddr, repoName str
 		"repo_name":  repoName,
 		"service":    fmt.Sprintf("%d", structs.PlainGitService),
 	})
-	resp = session.MakeRequest(t, req, http.StatusSeeOther)
+	resp = session.MakeRequest(t, req, http.StatusOK)
 
 	return resp
 }
@@ -40,5 +41,6 @@ func testRepoMigrate(t testing.TB, session *TestSession, cloneAddr, repoName str
 func TestRepoMigrate(t *testing.T) {
 	defer tests.PrepareTestEnv(t)()
 	session := loginUser(t, "user2")
-	testRepoMigrate(t, session, "https://github.com/go-gitea/test_repo.git", "git")
+	resp := testRepoMigrate(t, session, "https://github.com/go-gitea/test_repo.git", "git")
+	assert.Equal(t, "/user2/git", *test.ParseJSONRedirect(resp.Body.Bytes()).Redirect)
 }