Clean-up DateUtils.mm

[printercu]

• Remove duplicated if-branches.
• Use stringForShortTime to format time in all methods.
• Extract stringForDateLong method.

I have not changed any logic except removing if (timeinfo.tm_year != timeinfo_now.tm_year). This makes dates be formatted in consistent way on the first first week of year.

Indeed I want to add time to message timestamps as described in #32. But found that commit with some improvements got too large.

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