Events: Upload all images on MinIO S3

[Mulugruntz]

A follow-up to #28 .

The goal is to upload all images to our S3 bucket.

The scheme is event/<lang>/<YYYY>/<MM>/<filename>.

Example with this test image:
https://s3-api.dval.in/event/fr/2024/01/95de72899ba234747623aea05a36f796_5234945346975622605.gif

Also, at the same time, urlLocal should be removed and replaced by s3Resource, such as for the above example:

"s3Resource": "event/fr/2024/01/95de72899ba234747623aea05a36f796_5234945346975622605.gif",

I propose to not have the full URL in it, as changing the S3 DNS will mean that we'll have to change all the JSON's URLs.
But maybe that's not reasonable? As it would also make sense, in a future PR, to pre-compute the content with our S3. In that case, we'll need the full URL anyway.

If so, then we could have:

"s3Url": "https://s3-api.dval.in/event/fr/2024/01/95de72899ba234747623aea05a36f796_5234945346975622605.gif",