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
```

?