Change default comment visibility to expand all recent comments

Users had a hard time following change discussions, when all but the
most recent comment were collapsed on the change screen per
default. So we change the default to expand all comments that are not
older than a week.

Change-Id: Id2fc7c18e928331e66ac82c02d25b0483e1cd64f
Merge "Change default comment visibility to expand all recent comments" into wmf