Replace template_preprocess() and possibly other things with Twig global variables/functions

Problem/Motivation

For some time we've been talking about moving away from preprocess - via Twig autoescape and other things. This didn't really happen to the extent people hoped during the 8.x cycle though.

So at this point we have a lot of preprocess implementations, and a lot of templates relying on variables prepared by them.

We can't remove preprocess in 8.x, and we can't make variables that were previously available to templates unavailable.

However, we can potentially rationalize specific preprocess implementations, deprecate the use of particular variables etc.

template_preprocess() gets called for every single template render, to generate exactly the same variables to pass in - this would be a good candidate to remove.

Additionally we may be able to define twig variables as objects for other things, so that they're used on demand in the template, rather than always generating strings in preprocess as we currently do. Something like node $submitted would be a good candidate for that.

Proposed resolution

Replace the variables added by template_preprocess() with Twig global variables: http://symfony.com/doc/current/cookbook/templating/global_variables.html

Remaining tasks

User interface changes

API changes

Data model changes