Change timescale when grouping entries in list views

[passiomatic]

Current way of grouping entries with different publishing dates lead to lots of fragmentation for older entries. Also, it is not precise enough: sometimes you see posts slip into the "yesterday" group because current implementation of the time_since helper is buggy.

Instead, Coldsweat should have a more sensible timescale when grouping post dates in listings. I'm thinking about something like Instagram:

• Today (I see no reason to go below the 1-day granularity)
• Yesterday (today < date <= one day ago)
• This week (one day ago < date <= 7 days ago)
• 2 weeks ago (14 days ago < date <= 21 days ago)
• 3 weeks ago (21 days ago < date <= 28 days ago)
• One month ago
• ???

Not sure yet how to split these without ambiguity, though.