Support for hovercards (page previews) for wikilinks

[prototyperspective]

In Wikipedia, when you hover the mouse over a wikilink, it displays a small popup with the start of the linked article's lead. This is very useful and a key advantage of articles on Wikipedia vs on any other Wiki. It would make this Wiki way more useful if it could do the same thing. This may require #34 to get implemented first.

See https://www.mediawiki.org/wiki/Page_Previews