Nested content.jsons are not in sidebar.

### Step 1: Please describe your environment

* ZeroNet version: 0.7.1 (rev4253)
* Operating system: Fedora 30
* Web browser: Tor Browser
* Tor status: always
* Opened port: no
* Special configuration: -

### Step 2: Describe the problem:

Nested `content.json`s are not listed in the sidebar. under the `CHOOSE` section.
#### Steps to reproduce:

A new blank zite, the root `content.json` includes `data/content.json` with `includes_allowed` set to true.
`data/content.json` includes `L_admin/content.json`.

#### Observed Results:

The sidebar will not show `L_admin/content.json` or any other nested `content.json` that is not explicitly in the includes entry of the root content.json.

The problematic line is in [SidebarPlugin.py line 504](https://github.com/HelloZeroNet/ZeroNet/blob/py3/plugins/Sidebar/SidebarPlugin.py#L504):

```
contents += list(site.content_manager.contents.get("content.json", {}).get("includes", {}).keys())
```
It does not consider nested includes.

**Additional context**
This issue is mirrored from the [GitCenter ZeroNet repository](http://127.0.0.1:43110/1GitLiXB6t5r8vuU2zC6a8GYj9ME6HMQ4t/repo/issues/view/?1P4w4Rvh4vS9r6G5xTiSh8qEW87EKZtnJB/11@1DfrA2M9Qra6stqT6tzwNMYNweswY54KAC)(you need to install ZeroNet to visit this link)

Start in plugins/Sidebar/SidebarPlugin.py around line 504 and inspect how the sidebar builds the CHOOSE list from content.json includes. Reproduce the nested data/content.json and L_admin/content.json setup described in the issue. Done means nested content.json files appear in the sidebar without being explicitly listed in the root includes entry.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.