do not prepend asterisks before words in search

An asterisk is a special FTS4 operator when appended to a word but has no
special meaning when prepended.

src/main/java/eu/siacs/conversations/utils/FtsUtils.java

@@ -66,7 +66,7 @@ public class FtsUtils {
 			} else if (term.contains("*") || term.startsWith("-")) {
 				builder.append(term);
 			} else {
-				builder.append('*').append(term).append('*');
+				builder.append(term).append('*');
 			}
 		}
 		return builder.toString();