Enable JSON-format config file to be uploaded

[samueldobbie]

Config files are currently required to be in standoff format which is fairly niche. Allowing for JSON formats will allow for more universal usage.

[samueldobbie]

Considering a structure like this:

{
  "entities": [
    {
      "name": "Prescription",
      "attributes": [
        {
          "name": "unit",
          "values": ["mg", "g"],
          "allowCustomValues": true,
          "allowMultipleValues": true
        }
      ]
    }
  ],
  "global_attributes": [
    {
      "name": "date",
      "values": [],
      "allowCustomValues": true,
      "allowMultipleValues": false
    }
  ]
}