Add routes for API capture

[SimonLab]

To be able to test the capture feature (ie saving text in database) from dwyl/app#254 and https://github.com/dwyl/elm-pwa-example we need

• the /api/capture endpoint which will receive a post request with json data text

{
  "capture": "capture text..."
}

The logic of the capture api endpoint will be similar to:
https://github.com/dwyl/app-mvp-phoenix/blob/d746db8bb33d874f6433550b865d47162b554fc6/lib/app_web/controllers/captures_controller.ex#L12-L22

We need to make sure the Ctx module can handle json

• the /api/items/show endpoint which returns all the existing capture in a json format

https://github.com/dwyl/app-mvp-phoenix/blob/d746db8bb33d874f6433550b865d47162b554fc6/lib/app_web/controllers/item_controller.ex#L29-L32