Preparations for the upgrade to 2.5.5

[auge8472]

This PR contains the last fixes for the database updates and the common preparations for the upgrade.

• [x] Add the changes of the definitions for the database columns mlf2_entries.last_reply, mlf2_entries.edited, mlf2_userdata.birthday, mlf2_userdata.last_logout and mlf2_userdata.registered. These columns was defined with NOT NULL DEFAULT '0000-00-00( 00:00:00)' in ancient versions (previous to MLF 2.3.5) and later with NULL DEFAULT NULL but this change got never reflected in the update script. This will not change anything in installations with the newer definition.
• [x] Update the occurences of '0000-00-00' and '0000-00-00 00:00:00' to NULL in the affected database columns.
• [x] update the readme
• [x] update the changelog
• [x] update the version numbers in the files config/VERSION and index.php

[auge8472]

All things done. Waiting for the merge of the pull requests #724 and #725.