This is a simple change that introduces a new global-vars block in the

base template, which child templates can use to register global

variables (through the new {% definevar "..." global %} support in

Djblets). Any global variables defined here will be usable in any other

block in the template, and are a good place to define IDs, URLs, or

blocks of HTML that need to be used in multiple independent blocks.
The new block will be used in an upcoming change shortly.

Made use of this in an upcoming change. Verified that the variables were

usable in other blocks.