Remove XML-RPC and move to contrib module

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In Drupal 8, the XML-RPC system was moved from a stand-alone PHP file (core/xmlrpc.php) to a module (xmlrpc.module). I think that's a logical move, separating out functionality that is rarely used. However, the fact that it's rarely used makes it a candidate for removal. I can go both ways on this one, there's some preliminary conversation at this issue: #60 that was brought up because we were fixing a bug in XML-RPC.

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Relevant Drupal.org issues:

https://drupal.org/node/1285726 - Remove XML-RPC entirely (postponed)
https://drupal.org/node/1033818 - Move XML-RPC into xmlrpc.module (fixed)

And here's the current stand-alone D8 version of the xmlrpc.module:
http://drupalcode.org/project/drupal.git/tree/refs/heads/8.x:/core/modules/xmlrpc

[jenlampton]

Since this doesn't fall in our 90% use-case, my vote is to remove it entirely. It should live in contrib, not in core.

[mkalkbrenner]

I agree to maintain the xmlrpc module as contrib as soon as the most pending technical decisions are taken.

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Thanks! That's an excellent offer @mkalkbrenner! Now all we need is a place to centralize "contrib" ;)

There are a number of issues for getting backdropcms.org up and going: https://github.com/backdrop/backdropcms.org-issues/issues

One thing we're pretty confident about is that contrib module repositories will be hosted on Github. So when it comes time, step one is just to make a repository on Github. Longer-term, these repositories will be registered with BackdropCMS.org to provide a central place for browsing modules/themes.

I agree it's probably best to wait a little longer while the largest technical changes are being made. We're making fast progress already, but we have a lot of work to do.

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Merged in backdrop/backdrop#72.