Add the ability to define a threshold for available/partly/unavailable

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This threshold can be set on the resource:

available_threshold: if the allocation's availability is above or equal to this threshold, it's shown green
partly_available_threshold: if the allocation's availability is above or equal to this threshold and below the available_threshold, it's shown orange
unavailable_threshold if the allocation's availability is below this threshold, it's shown red

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Correction, the threshold must be set per site, in the seantis.reservation controlpanel. The problem is that the overview otherwise has to handle conflicting definitions.