Allow upload of (external) stand planning via JSON or some other format

[cpawley]

Some events (e.g. Speedbird 24) will have an external party organise a stand plan for the whole day. Would be handy to allow them to upload a JSON in advance and consult it for a period of time to assign the stands according to their plan (subject to usual occupancy rules)

[MrAdder]

@cpawley can you check the images within the PR and potentially the DOCS that have been added and see if this ticks the boxes, I might need to tweak the reservation process slightly and confirm it actually "reserves" the slots but need the outside perspective for the plan side of things