Drop support for remote volumes without CORS

[mzur]

As mentioned in #351 support for remote volumes without CORS should be dropped in a future release to avoid issues with the display of images. We could do this in an iterative approach:

• Show a prominent warning message for affected images.
• Show a notice when new remote volumes are created.
• Reject new remote volumes if CORS is not configured properly.

I don't think that we need to make the additional step and disable display of remote images without CORS entirely. The prominent warning message should make everyone aware of the issue.

[mzur]

I'll leave it at that and don't reject new volumes without proper CORS configuration. The warning message is enough. We won't drop support for these images either.