Links do not work in first Markdown heading of each cell

[stefanuddenberg]

This bug only manifests for the very first heading in any given cell, and only if the very first text in the cell is the heading in question. It does not matter what level of heading it is (so it could be sub-sub-sub heading and still yield the same problem). For example, the heading ## [Visit Google](https://www.google.com/) would appear as if it were a regular heading without a link, and would not be clickable — but only if it were the very first text to appear in the cell, regardless of where in the cell it appears.

(Tested on jupyterlab 0.35.4, Windows 10.)