apps/bplan: Replace imghdr with python-magic

[partizipation]

imghdr support was deprecated in python, so this uses python-magic and built-in mime to check the filetype of images files for bplans created using the API.

Also reorders some conditionals to fix a bug where bug tile_image was being used instead of image_url.

Test with curl like so:

curl  -X POST http://127.0.0.1:8003/api/organisations/1/bplan/ \
 --user '[email protected]':'password' \
 -H "Content-Type: application/json" \
 -d \
 '
 {
   "name":"Luisenblock Ost - Bebauungsplan 1-70",
   "bplan_id": "VI - 96a",
   "description": "Test",
   "url": "https://mein.berlin.de",
   "office_worker_email": "[email protected]",
   "start_date": "2019-01-01T00:00",
   "end_date": "2022-01-01T00:00",
   "image_url": "https://cdn.britannica.com/39/226539-050-D21D7721/Portrait-of-a-cat-with-whiskers-visible.jpg"
 }
'

The new bplan should be at http://127.0.0.1:8003/dashboard/organisations/liqd/bplans/

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