We have many cases where we need a unique ID for the purposes of the

aria-labelledby= or aria-describedby= attribute. In some cases, it's

a bit burdensome to have to come up with an ID, as templates are

sometimes generated dynamically based on iterating over other content.
This introduces a {% unique_id %} template tag that takes in a prefix

and outputs that prefix followed by an auto-incremented ID. It works

much like Underscore.js/Lodash's _.uniqueId(). function.
IDs are only unique if precautions are taken. It can't know how the ID

will be used or what else may be on the page. However, it won't generate

the same ID across different calls on the same template render.

Unit tests passed.
Made use of this in some in-progress code.