Use floating date-of-flight

[ericberman]

• date of flight resets (either due to choosing “reset” explicitly or due to submitting a flight) not to the specific date that is “today” but to an undefined date that floats.
• if you submit a flight with an undefined date, it is set to the current date. I.e., undefined= “local date that you submit the flight”
• the date displays “(today)” when undefined. Tapping it sets it to the current date, but it is now a firm (unchanging) date. I.e. if you do this on Monday and come back on Wednesday it still says Monday
• existing fail-safes (press and hold and setting a block out/engine start/ flight start) set it to a firm date that is “today”