FR: archived_before relative to current date

Setting user_contents.archived_before in content.json is not as useful as it might be.
It should allow to apply archiving (purging) in relative way.
Currently for site that user contents are no longer valid after period or for space limits concerns site owner have to manually apply purge dates.
New settings should allow automatic purging of user data older then number of seconds from current date.

Start by locating the handling of user_contents.archived_before in content.json and trace how purge dates are applied. Define the relative setting in seconds from the current date, then verify that user data older than that period is automatically purged while existing absolute-date behavior remains clear.

Written by the indexing model from the issue text.