Allow users to copy and paste images into the Upload Image modal

In this issue tracker, when you click the "Image" icon, you're presented with an option to drag an image file into the modal to upload it.

However, sometimes users don't want to save the image to their computer before uploading it. For example, maybe they used the snipping tool or took a screenshot and then cropped it in Paint.

It would be nice if the image upload feature allowed you to paste images directly from your clipboard, a feature that many image hosting websites have.

Relevant StackOverflow thread that contains a link to the clipboard API docs: https://stackoverflow.com/questions/6333814/how-does-the-paste-image-from-clipboard-functionality-work-in-gmail-and-google-c