Bug 1875492: Add 'start' and 'end' date options to the default group page settings

- added in start and end windows for default settings pieform
- alter date format to consider time as well as date
- alter calendar to accept an existing time as well as existing date
- added lang string for admin view

Change-Id: I4e2f781bf535fdd7f9f132cc0a6f273d7db13d00
Signed-off-by: Lisa Seeto <lisaseeto@catalyst.net.nz>