first working version:

using range to search for sessions

How it works:

1. first time page load: range start and end will be set to the current time with time zone UTC,
    all sessions going on at this time will be shown
2. when given a range:
    2.1 convert the given range with given given zone to a specific time zone (starting from UTC - 12 to UTC + 13)
    2.2 go to the datastore find all the sessions that overlap with this converted range
    2.3 loop 2.1 and 2.2 until all the time zones are checked