about:tor takeover for YEC '21

This ticket is to track the creative development, implementation and translation of the about:tor takeover for this year's (2021's) Year End Campaign.

For reference, the full task schedule is viewable here: Google sheets link

Current milestones as follows (subject to change):

• 9-9-2021: Present the initial designs to the YEC committee for review
• 14-9-21: Present the about:tor takeover to the YEC committee for approval
• 21-9-21: Deliver final strings for localization
• 28-9-2021: Deliver final assets for about:tor takeover
• 12-10-2021 19-10-2021: Current planned TB 11.0 release date

cc @sysrqb & @smith