Locust uses server time to store the time of a task in stats history.
When fetching the initial stats history for the client (such on on page
refresh) it is written by the server using using this stored time.
While rendering a chart on the client, locust uses the localized client
time for the time the stats history was fetched. This would lead to a
jump in the x-axis time series when server and client time zones
differed.

Stats history (stats.py) now derives time from the time component of
the current datetime in UTC instead of server time zone.
This aligns it with other time stores used throughout locust
- particularly the start_time and end_time of reports - that make use
of time.time(), which is in UTC.

The client UI report (report.html) now localizes the server-rendered
times from history (formerly in server TZ, now in UTC).
This aligns it with the client-side report timekeeping of locust.js
updateStats() used for the x-axis of charts.

Issue: locustio#1835

The "during" start and end datetimes of "Download Report" (/stats/report)
were using server-rendered times, i.e.: strftime("%Y-%m-%d %H:%M:%S").
When server and client time zones differed, the times of the report
would not match the client time zone.
These datetimes are now localized similarly to the rest of the report.

Simplify date parsing. The date value is converted to ISO8601-like
format in UTC (Z) and parsed using built-in Date.parse().