refactor: complete default values for global settings (#4562)

#### What type of PR is this?
/kind improvement
/area core
/milestone 2.10.x

#### What this PR does / why we need it:
补全系统全局设置的默认值

how to test it?
1. 初始化 halo
2. 安装评论组件
3. 到文章页面可以评论

#### Which issue(s) this PR fixes:
Fixes #4561

#### Does this PR introduce a user-facing change?
```release-note
补全系统全局设置的默认值
```

application/src/main/resources/extensions/system-configurable-configmap.yaml

@@ -6,7 +6,8 @@ data:
   user: |
     {
       "allowRegistration": false,
-      "defaultRole": ""
+      "defaultRole": "",
+      "avatarPolicy": "default-policy"
     }
   theme: |
     {
@@ -26,7 +27,17 @@ data:
     }
   post: |
     {
-      "review": false
+      "review": false,
+      "postPageSize": 10,
+      "archivePageSize": 10,
+      "categoryPageSize": 10,
+      "tagPageSize": 10
+    }
+  comment: |
+    {
+      "enable": true,
+      "requireReviewForNew": true,
+      "systemUserOnly": true
     }
   menu: |
     {