Fix #551

This change introduces the ability to 'transplant' a planting to a new garden.

A 'Transplant' option is now available in the actions dropdown on a planting's page. This allows a user to select a new garden from their list of active gardens.

When a planting is transplanted:

• The original planting is marked as 'finished'.
• A new planting is created in the selected garden, with all the attributes of the original planting.
• The user is redirected to the edit page for the new planting to allow for further tweaks.