How should variables be renamed in template files?

Part of meta-issue #1788918: [GTD] [META] Prepare for Twig main engine core inclusion until December 1

For now, let's leave all the variable names the same as they are - just to get things working. To do that, there are several good rules of thumb for renaming variables:

1) remove all dollar signs from the front of variables.
print $title; becomes {{ title }}

2) remove all square brackets and quotes (for arrays) and replace with a period.
print $content['links'] becomes {{ content.links }}

3) remove all arrows (for objects) and replace with a period.
print $node->nid; becomes {{ node.nid }}

Longer term, we do have a problem with inconsistencies of how variables are named.

Two problems we face are:

• inconsistent mixtures of data and attributes (see #1783156: [meta] Clean up inconsistent mixtures of data and attributes)
• should we name top level items? (see #1783136: should we (re)name top level data items in templates?)