week-grid: Convert event timezones to local on allocation

When inserting events into the display grid, use local start and end
times.

Fixes #599
2 jobs for master in 6 minutes and 35 seconds (queued for 1 second)
Status Job ID Name Coverage
  Flatpak
passed #714374
flatpak
flatpak:master

00:05:55

 
  Deploy
passed #714375
flatpak:nightly

00:00:40