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Currently for site that user contents are no longer valid after period or for space limits concerns site owner have to manually apply purge dates.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.

首先定位 content.json 中对 user_contents.archived_before 的处理，并追踪清理日期的应用方式。以当前日期为基准，用秒定义相对设置，然后验证早于该时间段的用户数据会被自动清理，同时确保现有的绝对日期行为仍然清晰。

由索引模型根据 Issue 内容生成。