Font-family of `add-tag` widget does not match `tag-holder` widget

[agoose77]

The <input> element does not inherit the style from the body. In addition, the font-size does not match that of the tag-holder widget.

I don't have many cycles to work on this right now, so I'm just filing the issue in case anyone has time to work on it before I do!

[fcollonval]

Thanks @agoose77

This may require some fix upstream in the JupyterLab widget:
https://github.com/jupyterlab/jupyterlab/blob/master/packages/celltags/src/addwidget.ts