Deprecate L20n in favor of Fluent

[stasm]

Project Fluent has been around for almost a year now.

• Website: http://projectfluent.org
• GitHub: https://github.com/projectfluent
• The original announcement: https://groups.google.com/forum/#!topic/mozilla.tools.l10n/NPmsJD4IGjQ

Let's make sure L20n is sunset properly and responsibly.

To do:

• [x] Edit the description of the L20n org on GitHub.
• [ ] Put a deprecation notice in this repo's README.
• [ ] Put a deprecation notice in l20n/l20n.js.
• [ ] Put a deprecation notice in l20n/python-l20n.
• [ ] Put a deprecation notice in l20n/l20n.org.
• [ ] Put a deprecation notice in l20n/tinker.
• [ ] Put a deprecation notice in l20n/builds.
• [ ] Put a deprecation notice in l20n/demo.
• [ ] Put a deprecation notice in l20n/preso.
• [ ] Put a deprecation notice in l20n/l20n-syntax-fixtures.
• [ ] Deploy a placeholder redirecting to http://projectfluent.org at http://l20n.org.
• [ ] Deploy a placeholder redirecting to http://projectfluent.org/play at http://l20n.github.io/tinker.
• [ ] Deprecate the l20n package on npm.

On 2018-11-26 we will let the l20n.org domain registration expire.