fetch available values for gallery via autocomplete method

We had a method that returned the values of a field
with respect to a query. That method was inefficient,
because it executed the query, fetched all Docs
and collected the values.
The autocomplete method we introduced a while back
can answer the same question but much more efficiently.

pdb-utils/src/main/java/org/lucares/utils/CollectionUtils.java

@@ -34,6 +34,17 @@ public class CollectionUtils {
 		return Stream.of(input).map(mapper).collect(Collectors.toList());
 	}
 
+	public static <O extends Collection<R>, T, R> O map(final Collection<T> input, final O result,
+			final Function<T, R> mapper) {
+
+		for (final T t : input) {
+			final R e = mapper.apply(t);
+			result.add(e);
+		}
+
+		return result;
+	}
+
 	public static <T, V> Map<T, V> createMapFromValues(final Iterable<V> iterable, final Function<V, T> keyMapper) {
 		final Map<T, V> result = new HashMap<>();