Change the way of time range in frontend and backend

The root cause is we use time diff for start_time/end_time originally,

and the time is different from browser(frontend) and API(backend), so there is delay in API (In my machine is 6 mins).

How to fix it:

I use UTC timestamp and add parameter timezone_offset to API to count the time range instead of time diff.

1. Modify chart.coffee file for start_time and end_time calculation and add parameter timezone_offset.

2. Modify widget-statistics.coffee file for modify usage of chart component. Remove time diff mechanism, just transfer datetime that user choosed to UTC timestamp. and calculate and set timezone_offset.

3. There are 2 type of chart: LineChart and LineChartWithCtrl, so I modified 2 places.

4. Modify ajax.py file for API changes. Calculate local time by timezone_offset that user gave(otherwise, 0 is UTC+0)

5. There are 2 type of API: ModelAjaxHandler.get_statistics_data and ObjectAjaxHandler.get_statistics_data so I need to modified both function.

6. I remove some space or useless code:

tmp_data = []

There is no place use this variable.