fix: avoid closing the script tag early by escaping a forward slash

[maartenbreddels]

Closes #1562
Closes #802
Related #804
Related spatialaudio/nbsphinx#183

Having a widget like this:

import ipywidgets as widgets
widgets.Label("Let me bug you with a closing tag </script>")

left an invalid HTML document, since there was a closing script tag that the browser directly parses, escaping the forward slash works around this, see:

• https://stackoverflow.com/questions/1659749/script-tag-in-javascript-string
• https://stackoverflow.com/questions/1580647/json-why-are-forward-slashes-escaped

[SylvainCorlay]

Elegant, simple, clean,

[mgeier]

It turns out that the tag name is case-insensitive and that browses also have a problem when the tag name is followed by a space (in this case it doesn't matter if a > is following or not.

Here's an example that makes problems:

widgets.Label('using </scRIPt can be problematic')

See also spatialaudio/nbsphinx#611.