Implement Issue Config (#20956)

Closes #20955

This PR adds the possibility to disable blank Issues, when the Repo has
templates. This can be done by creating the file
`.gitea/issue_config.yaml` with the content `blank_issues_enabled` in
the Repo.

tests/integration/api_issue_config_test.go

@@ -0,0 +1,52 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package integration
+
+import (
+	"fmt"
+	"net/http"
+	"testing"
+
+	repo_model "code.gitea.io/gitea/models/repo"
+	"code.gitea.io/gitea/models/unittest"
+	user_model "code.gitea.io/gitea/models/user"
+	api "code.gitea.io/gitea/modules/structs"
+	"code.gitea.io/gitea/tests"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestAPIReposGetDefaultIssueConfig(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	repo := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 1})
+	owner := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: repo.OwnerID})
+
+	urlStr := fmt.Sprintf("/api/v1/repos/%s/%s/issue_config", owner.Name, repo.Name)
+	req := NewRequest(t, "GET", urlStr)
+	resp := MakeRequest(t, req, http.StatusOK)
+
+	var issueConfig api.IssueConfig
+	DecodeJSON(t, resp, &issueConfig)
+
+	assert.True(t, issueConfig.BlankIssuesEnabled)
+	assert.Equal(t, issueConfig.ContactLinks, make([]api.IssueConfigContactLink, 0))
+}
+
+func TestAPIReposValidateDefaultIssueConfig(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	repo := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 1})
+	owner := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: repo.OwnerID})
+
+	urlStr := fmt.Sprintf("/api/v1/repos/%s/%s/issue_config/validate", owner.Name, repo.Name)
+	req := NewRequest(t, "GET", urlStr)
+	resp := MakeRequest(t, req, http.StatusOK)
+
+	var issueConfigValidation api.IssueConfigValidation
+	DecodeJSON(t, resp, &issueConfigValidation)
+
+	assert.True(t, issueConfigValidation.Valid)
+	assert.Equal(t, issueConfigValidation.Message, "")
+}