Remove begin_time and end_time from date records.

We'll find a new home for timestamps in events.
Fix translations for displaying dates
Add 'Event' type to date subrecord.
Add an extra 'timestamp' field to the event schema.
Frontend changes for UTC timestamps
Fix repo name in selenium test
EAC export fix: don't blow up if a non-system event is linked to an agent being exported.
Added test for EAC that I broke accidentally
Allow BC dates in date subrecords by prefixing the year with a -
Merge pull request #392 from hudmol/date-utc-changes

Date utc changes