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.

routers/api/v1/swagger/repo.go

@@ -386,3 +386,17 @@ type swaggerRepoCollaboratorPermission struct {
 	// in:body
 	Body api.RepoCollaboratorPermission `json:"body"`
 }
+
+// RepoIssueConfig
+// swagger:response RepoIssueConfig
+type swaggerRepoIssueConfig struct {
+	// in:body
+	Body api.IssueConfig `json:"body"`
+}
+
+// RepoIssueConfigValidation
+// swagger:response RepoIssueConfigValidation
+type swaggerRepoIssueConfigValidation struct {
+	// in:body
+	Body api.IssueConfigValidation `json:"body"`
+}