Images in emails

[sylvinus]

Right now all images are blocked with the CSP.

We should support both external images (proxified, maybe approved by the user) and internal (cid:) images.

[mosa-riel]

Could we also use MailDomain.custom_attributes for _proxy_external_images: true ? I don't think we shuold be able to turn off the feature anyway.

Or are these attributes just not meant for these settings? Independent of that, the current backend would get an image proxy endpoint, and we rewrite images URLS then in the serializer.py.

It gives a lot of advantages by proxying images and it is not that complicated to build. I would be able to work on it.

EDIT: moved this setting to env.