Fixes stuff to do with social embeds! This does not change the intended contents of social embeds, but it does restore that intention, which was never ported to content functions correctly (dear god).

• Resolves Album social embeds fail when only a date is present #555 by reworking that whole content function.
• Fixes social embeds not having the "author" line (not reflecting the headingContent slot), by fixing the link to the separate oembed.json file not actually being generated.
  • For albums this is "(First group name) | HSMusic Wiki"
  • For tracks this is "(Album name) | HSMusic Wiki"
• Fixes track social embeds not having descriptions - this was just never ported(!?)
  • This includes artist and cover artist names.
• Fixes album "author" lines (headings) showing the group directory instead of name
  • This was never apparent because that heading line wasn't displayed at all

There are some supporting internal changes, too.

• WikiInfo.canonicalBase now always ends with a slash
• pagePathStringFromRoot is a new extra dependency and is literally the path to the page you are visiting, as a string, e.g. "track/sburban-jungle/".