Accessibility: Expand accessibility issue template

Created by: courier-new

As part of estimating the effort that auditing and fixing accessibility issues will require for our team in Q2, I pretended to walk through all of the steps required to audit one of the Batch Changes user journeys. I found, when pretending to submit a GitHub issue for an accessibility bug, that I wanted to have a couple other pieces of information included here. I've modified the GitHub issue template slightly to reflect these fields, as well as more explicitly encourage setting the appropriate labels on the issue.

Test plan

This just modifies an issue template.