Important: #EXTBLOG-70 - Introduce publish date

See https://jira.typo3.com/browse/EXTBLOG-70

Description

This patch introduce a new field "publish_date" which replace the usage of crdate.

An UpgradeWizard copy the existing crdate value into the new field.

Impact

An existing installation must run the UpdateWizard, else the output is broken.

If use your own templates, ensure to use {post.publishDate} instead of {post.crdate} in your templates.