Fixes vector-im/riot-web/issues/11608.
This patch:
1. Turns the container of reaction buttons into a toolbar.
2. Makes each button span into a button with a tabindex and an aria-label.
3. Constructs an alternative label that differs slightly from the text displayed by the tool tip:
* It uses the names of the people who reacted.
* It puts a space before the "reacted with" text.
* It uses the actual emoji characters, not the converted colon-delimited shortNames, because the emojis usually tell blind users more about the expression.
* It omits the number of reactions, since that information is already conveyed by the names.
Signed-off-by: Marco Zehe <marcozehe@mailbox.org>
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