feat(seo): add meta keywords for supported languages

[srl295]

• keyboard-details.php: use the langauge name(s) to produce keyboards.
• template.php: support 'keywords' to pass to head()
• Head.php: to support keywords as a parameter to head(), rendered as a meta tag. Escape ASCII quotes properly as entities.

Note

This is a little contrived, but showing that the machinery is quoting properly, if the
keywords had the text Some "information" for <you>:

<meta name="keywords" content="Some &quot;information&quot; for <you>, Amharic Keyboard, Basketo Keyboard, Bench Keyboard, Dirasha Keyboard, Dizin Keyboard, Geez Keyboard, Gumuz Keyboard, Konso Keyboard, Suri, Tirmaga-Chai Keyboard, Tigré Keyboard, Zayse Keyboard, Multiple languages Keyboard">

yielding (showing proper unescaping)

> document.getElementsByTagName('meta')[1].getAttribute('content')
'Some "information" for <you>, Amharic Keyboard, Basketo Keyboard, Bench Keyboard, Dirasha Keyboard, Dizin Keyboard, Geez Keyboard, Gumuz Keyboard, Konso Keyboard, Suri, Tirmaga-Chai Keyboard, Tigré Keyboard, Zayse Keyboard, Multiple languages Keyboard'