Fix endorsement sources opening twice

Clicking either the ℹ️ link or the “Go to endorsements source” button would open the source twice, once from the native behavior of <a> or <button>, and once from the event propagating up to the onclick={this.handleOpenEndorsementsLink} listener on the <tr>. Use stopPropagation to avoid this (in the same way it’s already used for the organization name link).