overhaul of mailing lists section
overhaul of mailing lists section

- put them in a better order
- revised wording for list descriptions, including incorporating the
  best bits of the mailing list descriptions that were removed from
  the Code of Conduct page by
  https://github.com/scala/scala.github.com/pull/455
- add the no-job-listings blurb from the CoC page, removed
  by the same PR
- don't list scala-xml anymore, it's pretty dead
- move the Gmane links to the end, for a less cluttered look

andy1138/scala-lang,jeantil/scala-lang,jeantil/scala-lang,andy1138/scala-lang,jeantil/scala-lang,andy1138/scala-lang,jeantil/scala-lang,andy1138/scala-lang