Improve social-sharing text

[kwalrath]

Currently, when you add the URL for a page to a social sharing site (twitter, Slack, Facebook, G+, ...) every page has the same generic text and image. For example, here's what happened when I shared the glossary on Slack:

We can do better by specifying meta tags. For example:

  <meta name="description" content="Brief definitions of the most important words in the Angular 2 vocabulary">

I'm not sure how exactly we'd implement this, but it doesn't seem difficult.