Add Archive.org integration

 * adds an "archive" link next to permalinks, linking to the last version of the page on archive.org
 * Thanks to @qwertygc in https://github.com/shaarli/Shaarli/issues/38

We probably want this as a plugin (it adds an html element and a few css rules) so this needs work.
We probably want to disable this for private links by default (don't want to archive private stuff by accident)
We probably want to make the base url (https://web.archive.org/web/) configurable so that people use their own archivers)