Added various fields to garden

- active (default: true)
- location, latitude and longitude (because when you move house, you
    don't take your garden with you)
- area and area units (square feet or metres)
Added new fields to garden form
Added new garden fields to gardens/show
Added active/inactive scopes
Added new fields to gardens index
Only list active gardens in planting form
Don't allow planting in inactive gardens
Default garden location to the owner's location, if known
Set default garden locations

Also created a handy rake task to depopulate Null Island (i.e. fix up
anyone's lat/long that's somehow ended up as 0,0).
Added owner link to the garden page
Fixed broken test
actually this is a slightly better fix
Merge pull request #333 from Skud/garden-inactive

New fields for gardens