This adds a new option to {% definevar %} that allows a variable to be

registered in the top-most template context, ensuring the variable

survives past the current scope. This is useful for defining something

within a template that's intended for use in several distinct blocks,

such as a unique ID or URL that has to be referenced in several places.
It's recommended that consumers have a standard block in a base template

defined before any other blocks or logic that is reserved for global

variable definitions, in order to ensure that these variables are

defined before they're used.

Unit tests passed.
Successfully used this with some upcoming work in Review Board.