[meta] Convert everything in theme.inc to Twig

More general title. Updated summary.

Just an FYI here, I'm making some updates to the Twig sandbox that affect theme.inc, review the git logs from today or ping me in IRC if you have questions.

Also, this issue may have some big implications #1778610: Remove the check for a link template from l(), have l() always output just a string.

In the sandbox, I moved link.html.twig to link-default.html.twig because if it exists, it will turn on $use_theme in l() in common.inc. That feels like a hack to me, but we should follow-up in that other issue.

Based on #1905584: Move base theme system templates into /core/templates it seems this issue would be accommodated by #1898454: system.module - Convert PHPTemplate templates to Twig, though that may result in a large patch.

Ignore what I said about system.module in #6, we'll proceed to convert this separately of system.module and put in /core/templates as described in #1905584: Move base theme system templates into /core/templates

Adding more sub-issues

Adding more sub-issues

Created sub-issues for the relevant theme functions, and added notes about the ones I didn't create issues for.

I can't make heads or tails of this in drupal_common_theme():

function drupal_common_theme() {
  return array(
  …
    'help' => array(
      'variables' => array(),
    ),
  …
  );
}

There's no theme_help() and of course no help.tpl.php, so what is this doing, and is it something we need to convert to Twig?

Add theme_datetime() issue

Bullets

Adding issue about where these templates will live eventually

@Cottser #10 We're bound to find some odds & ends cleaning out the attic. This seems like a good example. If we can't find a implementation/callback, let's remove it as part of the conversion.

Add instructions as to where the templates go

Add note about how the sandbox relates to these issues.

Add more about theme functions not yet converted in the sandbox

Thanks to everyone who's worked on these issues!

I just created issues for region.tpl.php, page.tpl.php, and html.tpl.php, those are up for grabs now :)

I also created a postponed issue for theme_link(), I think we'll need to do something there after #1922454: Gut l(), fix theme('link'), have a working framework for consistently generated links inside and outside of Twig is resolved.

And I did a bit of detective work about #10 and created an issue to remove that:

#1961886: Remove 'help' definition from drupal_common_theme()

Adding active issues for converting region, page, html templates

Add postponed theme_link() issue.

Updated issue summary.

Fixing li tag