[Orgmode] FR: org-timestamp-up/down jumping configurable number of minutes

[Adam Spiers]

I never need to insert timestamps with a granularity of smaller than 5
minutes, and hardly ever smaller than 15 minutes.  So it would be
really nice if when over the minute portion of a timestamp,
org-shiftup/down increased/decreased the minute value in multiples of
5, 15, or whatever other value the user wants to configure.  So for
example, that way I could postpone the start/end of an appointment by
15 minutes with a single key chord.

For backwards compatability, the granularity could still default to 1
if that's the behaviour the majority of people prefer.