Update test suite to cope with Unsafe Browser homepage changes

wiki/src/misc/unsafe_browser_warning.html has been changed today on master and devel, in a way that sometimes breaks the test suite.

We need to:

1. Merge master into stable.
2. Fork a branch off stable.
3. On that branch, update features/images/UnsafeBrowserStartPage.*.png.
4. Ensure this fixes the problem: the “The Unsafe Browser can be used in all languages supported in Tails” scenario should pass without leaving fuzzy Sikuli candidates behind.
5. Review and merge into stable.
6. Merge stable into devel.
7. Update the pictures again on devel: the page is rendered differently on Buster.
8. Ensure this fixes the problem on devel too: the “The Unsafe Browser can be used in all languages supported in Tails” scenario should pass without leaving fuzzy Sikuli candidates behind.

Feature Branch: test/17004-unsafe-browser-homepage-stretch, test/17004-unsafe-browser-homepage-buster

Related issues

• Blocks #16209

Original created by @intrigeri on 17004 (Redmine)