add image up- and download endpoint

[jarkenau]

@lucaslootalot I created a first draft of the endpoint for uploading and downloading images, pls take a look at it.

Available at: gardens/<int:garden_id>/image

• POST

  Request:

  {
  "image":"data:image/png;base64,iVBORw0KGg ...",
  "coordinates":[
    {
       "name":"topLeft",
       "latitude":52.31703822683545,
       "longitude":7.630155086517335
    },
    {
       "name":"topRight",
       "latitude":52.31703822683545,
       "longitude":7.630155086517335
    }
  ]

  Response: 201 Created

• GET

  Request: Empty Body

  Response:

  {
  "data":"data:image/png;base64,iVBORw0KGg...",
  "coordinates":[
    {
       "name":"topLeft",
       "latitude":52.31703822683545,
       "longitude":7.630155086517335
    },
    {
       "name":"topRight",
       "latitude":52.31703822683545,
       "longitude":7.630155086517335
     }
   ]
  }

[lsiebels]

The JSON and the response should fit like this. When the PR is through, I'll put in the API and get back to you if there are any problems! 😄