RFR: JDK-8241030: rename HtmlTag to TagName

[Jonathan Gibbons]

Please review a mostly-trivial rename of HtmlTag to TagName, as 
discussed after the recent review for updating HtmlTree.

The rename was a single automated IDE operation.

In addition, some minor functionality on HtmlTag is moved to HtmlTree, 
which is the only place where it is used, while writing out the tree.

And, I have added additional links to pages on html.spec.whatwg.org and 
www.w3.org/TR/html51 to some classes in formats.html.markup. There are 
no manual changes to the classes in formats.html package.

One test was affected, that used the internal HtmlTag/TagName class. 
Apart from that, no tests were affected.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8241030
Webrev: http://cr.openjdk.java.net/~jjg/8241030/webrev.00/index.html

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