Donation banner on about:tor page for 2017 campaign

We want to add a new donation banner to the Tor Browser startup page for our fundraising campaign in 2017. The design is currently under development, and when it is ready, this ticket will be for patching torbutton to add the banner.

Our old banners were #20414 (moved) (2016) and #17565 (moved) (2015). We can hopefully reuse some of the code again.