[FIX] web: datepicker: notify all datetime changes

Before this fix, there was an issue when you changed the value of
a datetime field by editing direclty the input (without using the
datepicker popup).
If this value was equal to the offset of your timezone,
the new value wasn't saved. The problem was that no event was
triggered by the 'bootstrap-datetimepicker' lib because it
considered that the old datetime and the new datatime were the
same...

In odoo JS, we always manipulate UTC moment objects and we
manually add the timezone offset at rendering (we mainly do that
for testing purposes). When the offset is added, calling format()
on the moment object doesn't convert the datetime to the browser
timezone because it's flagged as being UTC.

The datepicker lib creates moment objects in the browser timezone.
So comparing the ones it creates with the one given by Odoo is
like comparing apples and oranges, and it gives wrong results
when the datetimes differ by the exact timezone offset. So in this
case, no 'datetime change' is triggered by the lib.

We could fix this by wrapping moment objects into some kind of
OdooDate class and stop manually adding the timezone offset, but
this would involve a refactoring that can only be done in master.

Instead, we prefered to patch the lib to ignore the comparison
between UTC and non-UTC objects, as this is the safest way we
find to fix this issue in stable version.