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)