Only show cell border when cell toolbar is hovered

[JasonWeill]

Problem

Since the cell toolbar was added, hovering over a rendered Markdown cell displays a gray border. The first cell is auto-selected on page load and its border is visible.

Proposed Solution

Only display a gray border around the cell when the user hovers over or focuses a button in the cell toolbar. If a user hovers over or focuses other elements in the cell, do not display the gray border.

Additional context

Code added in #12028

@williamstein notes that Colab also has an outline on the focused markdown cell.

[JasonWeill]

@aiqc notes that the first cell appears to be auto-selected on page load

[williamstein]

As noted, both cocalc and colab have an outline around focused markdown cells:

CoCalc

Colab

[krassowski]

This seems like a good candidate for a quick survey among users to see whether they have a preference. I do not mind the extra border, some users might actually like it.