Reset the position of the toolbar for compat with Gutenberg 15.7+

[ryelle]

See #176

This switches the toolbar to be static-positioned on wp.org, so that it shows up above the post editor rather than floating outside it. It does remove the "fixed" aspect, if you write a long topic, the toolbar scrolls off screen.

This is really just a workaround for wordpress.org, so that we can unpin the version of Gutenberg — it doesn't do anything to fix the issue anywhere else the editor is used. This could be ported as a quick fix for other editors, or (more likely) it needs a better solution. So feel free to take or leave this PR 🙂

Before	After

[johngodley]

Thanks! Yeah, I've been sitting on a similar change for a while and was hoping to come up with something better, but fundamentally a change will be needed in Gutenberg to support it.

I've put a similar change in Automattic/isolated-block-editor#230 and this will be included with #186