Revisit how dates and times are stored and displayed

[KimSJ]

1. It would be good to allow events to have different times on different days. I think "normal" entry should be a set of check boxes for days, with a start and end time for each selected day

2. It would be good to order events by start time DONE - Improve how events are ordered #189

3. it would be good to be able to filter events by timespan too.

4. I would like the possibility of a "freeform" time field, which if present would replace the automatically calculated display. Dangerous, in that they can get out of sync, so we'd need to prompt editors to examine and if needed correct the other if either is edited. (Only if freeform not-null, of course)
   Kind of thing I imagine would be the "real" timeslots, and a "This information will be displayed as: xxxx" with an [edit] button. Or if the freefrom exists, then "The automatic version of this information would be xxx; you are currently displaying yyy. [Use Automatic text] [Edit custom text] [OK].
   Example freeform:

   3rd -24th March. 9am - 6pm every day; late-night shopping Wed 'til 8pm

   Real timeslot days to be set up by site settings, with default times each day.

5. Site option to "allow any dates" in which case date/time entry becomes
   From date: to date: Start time: End Time:
   [add new range]
   Later lines override earlier ones' time setting, so if, for example, you have a two-week 9:00-17:00 period, you can add late-night shopping days, or half-day closing.

[KimSJ]

Might also be good to have site option to disble per-event times. For example, Open Studios have fixed open times, so you just need to tick the days you're open.