fix timezone handling

every date that comes from elasticsearch is UTC, parse it accordingly.
every date that gets passed around should be a DateTime object, or if it is a string, it must be a ISO8601 format including the timezone
the search resource now accepts strings with timezones, otherwise it will treat the string as being in UTC
the web interface internally only uses DateTime objects with timezones and converts to the local timezone at the last moment before showing it to the user

the elasticsearch query in the debug modal will show utc times, don't be confused

fixes Graylog2/graylog2-web-interface#518