Crop Timelines about cropplanning HOT 6 OPEN

This might be a second tab under the CropDB detail view.  A table would be 
created
(something like "CROP_EVENTS") with fields for:
 * event id (unique)
 * crop id
 * event name
 * approx weeks after planting (0 is week of, negative are weeks before, default sort)
 * approx time to complete
 * perhaps an "order" to clarify position (secondary sort)

When we need a timeline for a crop, we could select from this table with WHERE
crop_id = ... ORDER BY weeks_after_planting

We will also need to track which "date" the action refers to.  This will allow 
users
to enter, for example, "Prepare greenhouse for winter squash curing 1 week 
before
harvest" and will avoid ambiguity about whether the planting date refers to 
seeding
or transplanting.