events with overlapping dates have problems displaying

[bridgetkromhout]

Expected behavior

All previous events no matter if they have overlapping dates (start or end) will appear on https://www.devopsdays.org/events/ and the front-page sidebar.

Actual behavior

Seattle 2016 and Kiel 2016 had the same start and end dates. To sidestep this bug in the past, we listed a different start date (artificially). However, now the logic uses end dates, so currently only Seattle 2016 (and not Kiel 2016) shows up on https://www.devopsdays.org/events/.

This was known to be an issue in the new theme on the sidebar with start dates but was resolved somehow (see #140). Not sure if problems still exist for the sidebar.

Reproduction Steps

Set two past events to the same end date. View https://www.devopsdays.org/events/ and see that one is missing.

Same end date for Kiel and Seattle 2016:

Different end date for Kiel and Seattle 2016:

[mattstratton]

FWIW, I gave seattle and hoofington the same start date in a local test, and they worked fine, so it doesn't affect the homepage grid or the sidebar.