Make sidebar donate link optional?

### Step 1: Please describe your environment

* ZeroNet version: 0.6.3
* Operating system: OS X 10.11, Windows 10
* Web browser: Safari, Chrome, Edge
* Tor status: not available on Mac/available on Windows
* Opened port: yes

### Step 2: Describe the problem:

As a site owner, I'd like to be able to remove the "Donate" link in the sidebar for my site.

There doesn't appear to be a way to do this through the ZeroFrame API, perhaps it could be added as an optional flag in content.json? Something like:

```json
"donate": false
```

or

```json
"hide_donate_links": true
```

?

Start by tracing the sidebar Donate link and how site settings in content.json are exposed through the ZeroFrame API. Confirm the intended configuration flag with the issue discussion, then verify that disabling it removes the link while leaving the default behavior unchanged.

索引モデルが issue の本文から書いたものです。