Improve event unlock flow

[vidapour]

duplicate of #924

[tabascq]

Proposal:

• Have an optional "Puzzle unlock time" on a puzzle
• Have an optional "Team start time" on a team

If no team start time: Unlock a puzzle when "Puzzle unlock time" has been reached
If yes team start time: Unlock a puzzle when "Puzzle unlock time" + ("Team start time" - "Event start time") has been reached

And learn how to make the date/time editors use local time instead of UTC, so we don't have to do mental math on times any more