Accessibility software and discourse

[neil (Neil Lalonde)]

@MarcoZehe

Just one nit: The Close button in the Sharing panel is the only one I found that is still missing the label.

I added an aria-label here too. The timing of these fixes is perfect because I was just about to fix these for someone who uses a screen reader.

Self-hosted instances that follow the tests-passed branch will get these fixes when they upgrade.