Fix enforced share expiration date to be based on share time

To test:
#### Enable "set expiration date"
- [x] Date picker shows correct range for new shares (min = today, max = not set)
- [x] Date picker shows correct range for existing shares  (min = today, max = not set)

#### Force expiration date
- [x] Date picker shows correct range for new shares (min = today, max = today + 7 days)
- [x] Create share, set its "stime" to 2 days before, date picker shows correct range (min = today - 2 days, max = today + 5 days)

Please review @schiesbn @icewind1991 @DeepDiver1975 @nickvergessen