Add models and endpoints for Experiments

[timcowlishaw]

Experiments are groups of devices which are being used as part of a single experiment.

An experiment has an optional start and end time, and is owned by a user - the user does not need to own the participating devices.

you get GET/PUT/PATCH/POST/DELETE to the /v0/experiments endpoint with json like:

    {
      "name": "test experiment",
      "description": "a test experiment",
      "is_test": false,
      "active": true,
      "starts_at": "2024-01-01T00:00:00Z",
      "ends_at": "2024-06-30T23:59:59Z",
      "device_ids": [ 123, 456, 789 ]
    }

[oscgonfer]

Final to-dos:

• [x] As discussed: active is derived from internal logic on whether there is an end date or not, and if now<end_date https://github.com/fablabbcn/smartcitizen-api/issues/346
• [x] https://github.com/fablabbcn/smartcitizen-api/issues/347