Nyzo 636: social meta tags

Nyzo version 636 (commit on GitHub) improves meta tags used to generate social share previews.

This version affects web rendering only.

An overload of the Head.addStandardMetadata() method that accepts both title and description was added. The overload that accepts only title passes through to the new overload, passing title for both values.

In addition to the og:title, og:description, and og:type fields that were previously set, the og:image fields are now set. The values for these fields are retrieved from the preferences file.

Metadata is also now added for a Twitter card. This uses the same image as the Facebook metadata. The twitter:site and twitter:creator values can both be overridden by corresponding values in the preferences file.