Label message edit field as such for screen readers

See https://github.com/vector-im/riot-web/issues/9747

src/components/views/elements/MessageEditor.js

@@ -187,6 +187,7 @@ export default class MessageEditor extends React.Component {
                     onInput={this._onInput}
                     onKeyDown={this._onKeyDown}
                     ref={ref => this._editorRef = ref}
+                    aria-label={_t("Edit message")}
                 ></div>
                 <div className="mx_MessageEditor_buttons">
                     <AccessibleButton kind="secondary" onClick={this._cancelEdit}>{_t("Cancel")}</AccessibleButton>

src/i18n/strings/en_EN.json

@@ -1052,6 +1052,7 @@
     "%(oneUser)schanged their avatar %(count)s times|one": "%(oneUser)schanged their avatar",
     "collapse": "collapse",
     "expand": "expand",
+    "Edit message": "Edit message",
     "Power level": "Power level",
     "Custom level": "Custom level",
     "Unable to load event that was replied to, it either does not exist or you do not have permission to view it.": "Unable to load event that was replied to, it either does not exist or you do not have permission to view it.",