Sync with string-meta and fix UTF-16 linking

[aphillips]

This PR depends on pushing w3c/i18n-glossary#74 to TR, since that PR contains the term 'string direction'.

• Remove links to UTF-16 or convert them to href links instead of glossary links. This was necessitated because we don't export that term any more.
• Fix two instances of UTF-16 being lowercase.
• Add the most basic lang/dir requirement from string-meta verbatim.
• Change instances of 'base direction' that refer to strings to 'string direction'. Note that the term 'base direction' still appears in specdev.

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