week-header: Clone and modify new event when dropping

Update week-header to use the same clone strategy used by month-cell
2 jobs for master in 7 minutes and 22 seconds
Status Job ID Name Coverage
  Flatpak
passed #769361
flatpak
flatpak:master

00:06:39

 
  Deploy
passed #769362
flatpak:nightly

00:00:43