Add more useful "last saved" relative time stamps

[hdodov]

Currently, the "last saved" text only displays relative time in minutes. That's pretty useless once a few hours have passed, especially days:

It'd be great if it works more traditionally and moves from minutes to hours, days, months, and years.

[DanRibbens]

I think this could be considered a regression, I'm pretty sure it was behaving the way you described. I remember trying to implement it this way when making the admin UI internationalized. If you're interested in making a contribution I can add some notes here on how to fix it.