Add vivaldi-scheme as a default in Whitelist (#1785)

Vivaldi is another browser based on Blink, that supports uBlock. Adding its scheme in the default whitelisting would prevent any issue with it.

src/js/background.js

@@ -80,6 +80,7 @@ return {
         'chrome-scheme',
         'loopconversation.about-scheme',
         'opera-scheme',
+        'vivaldi-scheme',
         ''
     ].join('\n').trim(),