[RCD] Elastic skin ready for testing

[A.L.E.C]

Yesterday I merged the separate repository we had for Elastic code with the main Roundcube
repository. You might notice ~530 new commits. So, the skin is now official.

We're not yet ready for the beta release though. I think it will happen somewhere in the
end of Q2. But if you're a plugin author it's time to start working on the new skin support.

There are still things that may change according to the feedback we collected at
https://github.com/roundcube/elastic/issues and https://git.kolab.org/tag/ux_seminar_ws17/.

ps. it uses Less, so it does not work out of the box. Please, read README for simple
installation instructions or use devel_mode with performance degradation.