Accessibility: send accessibility announcement events as confirmation for some actions

[codeofdusk]

Especially after #953, it may not be obvious that some actions were successfully performed. Even without it, double-tapping on a comment (by default) toggles its collapsed state, but the fact that this has occurred isn't immediately obvious without rereading the comment.

The app should send brief announcements indicating the success of some common actions:

• upvoting
• downvoting
• unvoting
• expanding comments
• collapsing comments

It looks like events can be sent like this, but I'm not sure at exactly what point in the code it would be best to implement this.