Make reaction buttons more accessible

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>

src/components/views/messages/ReactionsRow.js

@@ -149,7 +149,11 @@ export default class ReactionsRow extends React.PureComponent {
             </a>;
         }
 
-        return <div className="mx_ReactionsRow">
+        return <div
+            className="mx_ReactionsRow"
+            role="toolbar"
+            aria-label={_t("Reactions")}
+        >
             {items}
             {showAllButton}
         </div>;