LP#1880028 Backdate Checkins Until Logout Option

LP#1880028 Backdate Checkins Until Logout Option

- Adds a checkbox option to store the backdate as a
  session item and only loads the backdate on the checkin
  screen (backdate ignored on the hold capture screen)
- Adjusts existing $watch to enable/disable the checkbox
  and store/remove session backdate when appropriate
- Adjusts is_backdate() to not consider null or undefined
  dates as backdates so the visible alert is accurate
- Adjusts "today" comparison to use a time of 00:00:00 so
  that when the Clear button is used and clears the time
  the Today button won't cause a backdate just because it's
  an earlier time on the same date

Signed-off-by: Dan Briem <dbriem@wlsmail.org>
Signed-off-by: rfrasur <rfrasur@library.in.gov>
Signed-off-by: Dawn Dale <ddale@georgialibraries.org>
Signed-off-by: Jason Boyer <JBoyer@equinoxinitiative.org>