Make the warning of about:config not skippable

We think we could force release users to always see the about:config warning, to remind that it can be dangerous.

We thought of:

• always show the warning (by locking browser.aboutConfig.showWarning = true)
• remove the warning checkbox
• spruce up about:config interstitial (FF example below)
  • use TB colors?
  • change warning to at least mention fingerprinting (and drop performance to keep it short)
  • maybe a learn more to the about:manual entry
  • we need some RED somewhere ... RED WINDOW OF DEATH sounds good

from #41797 (comment 2934868)

cc: @pierov @donuts