Revert "Show burndown charts with User timezone dates" Fixes #815

This reverts commit a462a134a34765cf7e1a2543de7e5060cbaa893c.
Reason: using user's timezone here merely influences the Graph legend.
If you happen to cross date boundary against server time, the date
names just shift and display irritating information.
timezone for burndown, explicitly explain what happens with dates/times on burndown

settings page explanation text
sanitize Time.zone in before-test
Merge branch 'feature/burndowntimezone'