Add proper timezone support

[marksteward]

To be done by 29 March.

Use aware timezones for input data possible, and check that they're not ambiguous:

https://github.com/pubstandards/pubstandards-london/blob/master/tests/test_ps_data.py#L71

Output timezone to the ical, and add the timezone to each event.

[marksteward]

WIP on https://github.com/pubstandards/pubstandards-london/tree/timezone_support, and currently running at http://ps.rack.ms/.

Currently doesn't include a VTIMEZONE block, and (perhaps because of that) doesn't seem to load in Google Calendar. works in Google Calendar now.

[marksteward]

To do:

• VTIMEZONE blocks in ICS (there's code linked from ignores VTIMEZONE collective/icalendar#44) hopefully not needed, but moved to Add VTIMEZONE support #19
• Start/end times in json for SubStandards events where known moved to Add times for past Substandards where the start/end time is known #26
• Make sure all web pages track through with aware timezones, and add tests for visiting in/out of DST
• Consider removing the timezone override field. It doesn't remove much code, but it strengthens the assumption of only one event per day.

[marksteward]

I've made sure there's some testing of the queries made in the web pages. We don't yet have any end-to-end template tests.

[marksteward]

icalendar now supports VTIMEZONEs as of 24th March, so this could be easily added now.

So when they say "fully supported in icalendar for serialization and deserialization" they mean "if you create a Timezone object with all the necessary transitions", rather than exporting from pytz.

Full VTIMEZONE support moved to #19.