Add the changelog entry for the world clock and timeline

changelog.d/1512-world-clock-timeline.md

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+- One world clock now drives everything with a time of its own: the sky,
+  the HUD clock, satellites, launches and weather. A timeline along the
+  bottom of the screen, shown while one of those layers is on, plays, pauses
+  and scrubs it, with a tick for each launch or frame and a way back to
+  live. Share links made away from live carry the time (samehkhamis, #1512).