We need a notice to pop up when images aren't showing

[kyrieb-ekat]

Made of two parts:

1. A banner with an alert the user must click to dismiss, saying something along the lines of "We're aware of the issue if you are experiencing difficulty viewing images..." (etc).
2. this banner should be attached to a watching something that will let us know when images aren't showing up and will notify us, and trigger the banner.

This was started in #938 to explicitly pop up on Salzinnes (which was killed due to its overall resolution occurring via #936), but this should be a system wide feature.