Replace post parameters by JSON input

[krzd]

This will clean up the codebase a lot, potentially make it faster to run and implement changes, and it could make it easier to implement validation (see other issue).
Unless there are any objections, this would be part of a series to simplify the server code, make it faster and more reliable. For example we might also want to reduce the number of database queries and subqueries etc. That potentially also reduces the memory consumption. Plus right now for others to maintain it is a pain, because even I start struggling with my own code.
Because those will be quite a lot of changes, it would run in parallel to the current server API (versioning is already implemented). So older version of the app would use the old implementation, whilst newer will use this improved one, until the older versions are not used anymore.