a11y: Semantics for reaction chips in the message list

[chrisbobbe]

We should give appropriate direction to screen-reader software (such as Apple's VoiceOver) in the area below a message that shows message reactions, in the message list.

Probably the user should be able to select that whole area and hear "Message reactions"—or perhaps that plus e.g. "(10 total)"—then be able to focus each chip in turn, hearing e.g. "[emoji name]: you and 3 others: [list of names]". Then there's probably some structured way to tell the software that it's a button, along with (hopefully) what will happen if it's pressed; and the screen reader will turn that into the right speech for the user.