Display warning that only one image can be uploaded in the images menu after multiple images dropped into a markdown editor

[MisRob]

U## Summary

Currently, drag&drop is allowed, but when a user drops two images at once, only the first one will be uploaded. There is no warning.

Solutions:
1. Disable drag&drop
2. Leave drag&drop enabled and allow more images to be dropped (in that case we would probably need to use option (2) when dealing with #2177)
3. Leave drag&drop enabled, allow one image to be dropped and add some logic to communicate it to users - displaying a dialog with a warning that only a first file will be processed could be a simple solution?

@jtamiace @jayoshih Any thougths?

At first, implement #2177. Make sure that "You can only upload one image at a time" warning is displayed according to the design @jtamiace uploaded.

[MisRob]

Prerequisite issue #1853

[jtamiace]

I made a design for the third one in the specs - let me know if this works out okay

[MisRob]

Yes, it makes sense if we use 3. solution for #2177

[MisRob]

Description updated to reflect the decision.

[MisRob]

#1853 has just been closed (the image upload is working) so this should be ready to be addressed.

[MisRob]

Regarding dropping more than one image right into the editor's content area, I haven't found a way to distinguish this scenario from when only one image is dropped because TUI's addImageBlobHook provides the first file only even when more files are dropped.

There's a TUI issue already. It might make sense to wait and then implement this UX, so it works for drop action properly too.