Support comma-delimited string as labels in issue template (#21831) (#21873)

Backport #21831.

The [labels in issue YAML templates](https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/syntax-for-issue-forms#top-level-syntax)
can be a string array or a comma-delimited string, so a single string
should be valid labels.

The old codes committed in #20987 ignore this, that's why the warning is
displayed:

<img width="618" alt="image" src="https://user-images.githubusercontent.com/9418365/202112642-93dc72d0-71c3-40a2-9720-30fc2d48c97c.png">

Fixes #17877.

templates/swagger/v1_json.tmpl

@@ -16806,11 +16806,7 @@
           "x-go-name": "FileName"
         },
         "labels": {
-          "type": "array",
-          "items": {
-            "type": "string"
-          },
-          "x-go-name": "Labels"
+          "$ref": "#/definitions/IssueTemplateLabels"
         },
         "name": {
           "type": "string",
@@ -16827,6 +16823,13 @@
       },
       "x-go-package": "code.gitea.io/gitea/modules/structs"
     },
+    "IssueTemplateLabels": {
+      "type": "array",
+      "items": {
+        "type": "string"
+      },
+      "x-go-package": "code.gitea.io/gitea/modules/structs"
+    },
     "Label": {
       "description": "Label a label to an issue or a pr",
       "type": "object",