Various fixes for cartes.app integration

• remove user-agent from us doc: fixes #363 (closed)
• Add a non stac shortcut for pictures PATCH: fixes #362 (closed)
• Add a way to specify semantics at upload

It is a bit more complex than at patch because the upload is done with multipart, so the semantics should be a multipart json field, with content-type application/json.

The curl is like:

curl -X POST https://my-panoramax.fr/api/upload_sets/1234/files \
	-H "Authorization: Bearer YOUR_TOKEN_HERE" \
	-F file=@my_picture_001.jpg \
	-F 'semantics=[{ "key": "osm|amenity", "value": "post_box" }];type=application/json'

Note: with the semantics at upload time it was not mandatory to have the non stac aliases, but it was easy to add and could help others.