Notes: Allow downloads from widgets.
We are working on a widget that allows the user to download a file (a ICS calendar entry). Right now the sandbox of the widget iframe doesn't allow downloading. Instead, the following error is displayed in the console (for Google Chrome):
```
Download is disallowed. The frame initiating or instantiating the download is sandboxed, but the flag ‘allow-downloads’ is not set. See https://www.chromestatus.com/feature/5706745674465280 for more details.
```
Therefore this PR adds `allow-downloads` to the sandbox capabilities.
Steps to reproduce:
1. Create a simple widget with an `index.html` file like, e.g.
```
<a href="index.html" download>Download</a>
```
2. Host the widget somewhere, add it to the room and open the widget
3. Click on the download button
* Without the fix: Nothing happens, there is a warning in the console (see above)
* With the fix: The file is downloaded
Signed-off-by: Oliver Sand <oliver.sand@nordeck.net>
2e6f616e91