Date-based generic views mix dates and datetimes unsafely

[Aymeric Augustin]

Date-based generic views can use either a DateField or a DateTimeField. But the logic of the lookups to handle these two cases is a bit fuzzy. When time zone support is enabled, it isn't correct (or at least, not obviously so).

In order to make the code less fragile, I rewrote much of the interval handling logic.

This patch:

• adds some explicit conversions between dates and datetimes, accounting for the current time zone,
• makes the code generally clearer and possibly more correct — off-by-the-UTC-offset bugs are less likely when USE_TZ = True.

As a side effect, this patch is likely to fix two bugs:

• an egde case where accessing next_day/week/month/year on a DateTimeField could 404 even with allow_empty = False due to approximative filtering,
• #17192.

[Aymeric Augustin <aymeric.augustin@…>]

In [20e697368219603599649bc67aea5e087caedeae]:

Fixed #18323 -- Refactored date arithmetic

in date based generic views, in order to deal properly with both
DateFields and DateTimeFields.