Add hidden heading for "Key Points"

[rgaiacs]

Some callout blocks will automatically create a heading but the "Key Points" does not create a heading. I think that a automatically "Key Points" heading will improve the accessibility of the lessons, specially for readers with visual impairments.

Before	After

1st Solution

Instead of

<span class="callout-header">Key Points</span>

use

<h2 class="callout-header">Key Points</h2>

2nd Solution

Create the heading inside

<div class="callout-content">

and use CSS like

position: absolute;
width: 1px;
height: 1px;
overflow: hidden;

to hide it.

[froggleston]

This will need a modification to the lua filter specifically for the key points callouts, as the key point section is the same as other callouts in that it uses a span for the callout title.