'prehistory' release isn't mandatory
'prehistory' release isn't mandatory

The first record date is used as first release start date, then
having an ignored 'prehistory' release isn't required anymore.
The previous behavior is kept: 'prehistory' release is ignored and
related records aren't displayed.

Change-Id: I9460cc37c92eddc738ddbe5a0da5e17475b45636
Signed-off-by: Pierre-Louis Bonicoli <7a7613ec9cdf8183c029137b52ab44c195e3ab05@libregerbil.fr>

stackforge/stackalytics,0xf2/stackalytics,0xf2/stackalytics,openstack/stackalytics,stackforge/stackalytics,openstack/stackalytics,openstack/stackalytics,0xf2/stackalytics,0xf2/stackalytics,stackforge/stackalytics,openstack/stackalytics