downloadModel API call with polygonString should allow including/excluding border reactions

Currently the API call to download a diagram part by polygon:

$ curl 'https://minerva-service.lcsb.uni.lu/minerva/api/projects/test_project/models/670:downloadModel?handlerClass=lcsb.mapviewer.converter.model.celldesigner.CellDesignerXmlParser&polygonString=0,0;100,0;0,100' -i -X GET

excludes border reactions, e.g. the ones that don't have all their elements inside the polygon. Currently to include those I have to:

1. Find elements in the crop rectangle of the polygon (finding elements inside the polygon itself is beyond me at this point)
2. Find reactions with at least one of these elements
3. download a diagram part that containing all elements of reactions from [2] using &elementIds (something is wrong with the &reactionIds, see #1768)

If the API call would allow a parameter (e.g. strictCutoff=true/false) this would be much easier.