Add a proxy url for the s3 media fetch

[Nastaliss]

We should add a configuration to allow using a proxy instead of the real s3 url. The s3 url is sometimes flagged as malicious in some entreprise networks.

• [x] #287
• [x] #293
• [x] #294

[Nastaliss]

This is now deployed to the production platform. Please keep in mind that to trigger a refresh of the alerts you need to empty your session storage and refresh the page

[Nastaliss]

The configuration is passed as an env variable S3_PROXY_URL to the api. It replaces the original url with this url when passed. By passing an empty string or no value the original behavior is kept. The url format is the same as the S3_URL, you must include the protocol (https:// / http://) and end with a /.

[frgfm]

To the best of my understanding, this is already taken care of :)