Fixes 3182

[withrocks]

Issue #3182 "Can't paste two images from clipboard in markdown cell"
happens when the user tries to paste a second image into a text
cell and the image gets the same attachment key as the previous one.

This patch fixes that issue by adding an index to each picture that's
pasted after the previous one. Example:

• User pastes an image blob A. The text ![image.png](attachment:image.png)
  is added to the text cell and the blob is saved as attachment image.png
• The user pastes a second image blob B. The same happens as with blob
  A, except the key is now image-2.png rather than image.png

Special case:

If the user pastes the exact same blob A twice, it will not be added to
the attachments, but the textcell will get a second entry of
![image.png](attachment:image.png).

[withrocks]

I wasn't able to read the general Jupyter general contribution guidelines because the link is broken: https://jupyter.readthedocs.io/en/latest/contributor/content-contributor.html

I hope that won't be an issue.

[kevin-bates]

Hi @withrocks - your PR content seems fine although I'm not qualified to review it.

The broken link should point here: https://jupyter.readthedocs.io/en/latest/contributing/content-contributor.html

I've opened issue #5599 for this.

[blink1073]

LGTM, thanks!