Improve preview for sharing on Facebook

[zoul]

This is how the certificate website currently looks when shared on Facebook:

This could use an improvement. It’s quite easy to change the displayed preview using some meta tags in the page header (see docs):

<meta property="og:title" content="website title">
<meta property="og:site_name" content="website name">
<meta property="og:description" content="short summary">
<meta property="og:image" content="thumbnail URL">

It would be nice to have the strings localizable.

[shevski]

Hey zoul, perhaps you'd like to send in a pull request for this?