Keep time zone information when creating widget

Absolute time ranges in widgets were getting converted into
ES_DATE_FORMAT, which converts the time into UTC, but loses the time
zone information from the string representation. As the AbsoluteRange
create method parses the date time and tries to keep the time zone
information, that resulted in UTC times being converted into local time.

To fix the problem we avoid using ES_DATE_FORMAT, and use ISO8601
instead.

Fixes #2428

graylog2-server/src/main/java/org/graylog2/timeranges/TimeRangeFactory.java

@@ -17,7 +17,6 @@
 package org.graylog2.timeranges;
 
 import com.google.common.base.Strings;
-import org.graylog2.plugin.Tools;
 import org.graylog2.plugin.indexer.searches.timeranges.AbsoluteRange;
 import org.graylog2.plugin.indexer.searches.timeranges.InvalidRangeParametersException;
 import org.graylog2.plugin.indexer.searches.timeranges.KeywordRange;
@@ -40,8 +39,8 @@ public TimeRange create(final Map<String, Object> timerangeConfig) throws Invali
             case "keyword":
                 return KeywordRange.create((String) timerangeConfig.get("keyword"));
             case "absolute":
-                final String from = new DateTime(timerangeConfig.get("from"), DateTimeZone.UTC).toString(Tools.ES_DATE_FORMAT);
-                final String to = new DateTime(timerangeConfig.get("to"), DateTimeZone.UTC).toString(Tools.ES_DATE_FORMAT);
+                final String from = new DateTime(timerangeConfig.get("from"), DateTimeZone.UTC).toString();
+                final String to = new DateTime(timerangeConfig.get("to"), DateTimeZone.UTC).toString();
 
                 return AbsoluteRange.create(from, to);
             default: