Add a button to the watchlist to delete all items in the watchlist.

This also fixed controllers missing important use statement.

Sorry about the TWIG file, the line endings were changed from CRLF to LF. The only change in that file is:
{% if watchlist %}
 <a class="" href="{{ path('watchlistremoveall') }}">
 <input type="button" value="Delete All Watchlist Items" class="btn" />
 </a>
{% endif %}