Add relesaes service to get layer releases timestamps

In RE2, we had the zeitreihen service to get 'release years' of the zeitreihen layer for a given zoom level and a given location. This PR adds a similar service in RE3. It's a little more generic than in RE2 and can be used on any layer (of course, AFAIK only zeitreihen has release years).

This is to be used in the new per-year prints currently under dev by @procrastinatio . As discussed, this new service will be used in front-end to get the desired timestamps for printing, which are then send to the print service.

@cedricmoullet As you are the author of the zeitreihen service of re2, can you please review by taking into account the following:
- This new service doesn't use meta models. In my opinion, we have everything we need in the existing zeitreihen models. Or were there specifics in the meta models that I'm not aware of?
- Instead of a point and a tolerance, this new service here takes a mapextend (meters) and a imagedisplay (pixels) to calculate the resolution/scale for the correct model. The idea is to get all releases for the selected rectangle in the print service