Create survey banner on about:tor for TB-Alpha

We're planning on deploying an iteration of the survey banner created in !34 (merged) on TB-Alpha for the upcoming Snowflake Client User Survey (ideally around Feb 23). It should be functionally identical to the banner being deployed on TB-Stable for the Tor Browser Usage Survey, save for the following differences:

Icon

• snowflake-survey-icon_2x
• snowflake-survey-icon_3x
• snowflake-survey-icon.svg

Text

Do you use Snowflake?

Snowflake is a pluggable transport available in Tor Browser. Help us improve it by completing this short survey about your browsing experience:

[Launch the Survey]

Link target

http://bogdyardcfurxcle.onion/index.php/491436?lang=en

(don't worry if that 404s, it's currently unpublished)

Full mockups of both banners can be viewed here too: https://www.figma.com/file/DPWy3cLbmLy0M7OQYlqAgG/About-Tor?node-id=0%3A1