Repair survey icon on Safest security level

Before you remove the survey banner ( torbutton#40031 (closed) ), please make sure the icon in the banner displays when the user has selected the Safest security level in torbutton.

Test:

1. Select Safest security level.
2. Navigate to about:tor.
3. Observe the empty image placeholder where the survey icon would be.

https://blog.torproject.org/comment/291208#comment-291208 https://blog.torproject.org/comment/291300#comment-291300

On about:tor, the image of the DuckDuckGo icon in the search bar displays fine in the Safest security level. But that image is a PNG whereas the survey icon image is an SVG which are disabled in Safest level.

resource://torbutton-assets/searchLogo.png resource://torbutton-assets/tor-survey-icon.svg