datetime: Fix handling of Irish timezone on map

The timezone map tries to highlight regions of the world that keep the same time, approximating this by their non-daylight-savings UTC offset. There's no reasonable API for this, and it goes wrong in various cases, such as Irish Standard Time which is legally defined as the country's summer time with a negative DST offset in winter.

Hardcoding this is unpleasant, but there doesn't seem to be a better solution, and in any case there's already similar hardcoding implied by the segmented map images in panels/datetime/data/timezone_*.png. I've tried to make it practical to fix other similar disagreements between the detected offset and the groupings implied by map images, though for now I've conservatively fixed only the case I'm familiar with.

Fixes: #1341 (closed)