Notebooks from Jupyter Lab have empty slideshow tag - they are not presentations

[Piotr]

In Jupyter Lab, if you are using slideshow cell types for one notebook (making presentation), and create some other notebooks, then other notebooks will have empty slideshow tag in notebook ipynb file. Because of this, Mercury will treat them as presentations.

Please add stronger check for notebooks to be treated as presentations. There should be at least one tag:

"slideshow": {
"slide_type":"slide"
}

Right now we only check if there is slideshow tag.