Cleanup special:interwiki intro

* Move all the legend container stuff into the system message and added a
  header to the table.
* Merge interwiki_intro and interwiki_intro_footer into a single message,
  rewrite the content to be more useful, and also show it all to
  non-privileged users.
* Move the log link into its own message.
* Move the add new link above the legend as well, directly underneath the log
  link.

Change-Id: I184182d3171f362f2e8470008f4ea7cda6629f80