create feed entries for last seven days and for next day

The feed entry for next day is only displayed if DJI value is already
available.
cache error responses from DJI requests for one hour

This should reduce load on DJI server when periodically requesting feed
entry for next day.
improve error response caching (don't keep asking for future data)

When requesting data for the future, only retry once a day (at 14:30 UTC,
when DJI should become available). OTOH reduce retry interval for past
data to 30 minutes.
improve estimation of DJI opening time

Instead of using 14:30 UTC as opening time, expect that the DJI value
will be made available at 09:30 New York time, and convert that to UTC.
Conversion is done in a quite hacky way, by temporarily changing the
timezone of the current process.
try to show geohash locations for next four days

This allows to display all hashpoints for a weekend. With the improved
error caching behavior, server load on the DJI server should still be
pretty low.
add 10 minutes to expected DJI opening time (to allow for some delay)