Minor tweaks to seed trading:

- changed "Will trade to" to "Will trade" on seeds/show page
- on crop page, only list tradable seeds (not "nowhere" ones) -- did
this by adding a tradable scope to Crop
- on crop page, say "(location unknown)" where necessary
- change heading on crop page to "Find seeds"
- show "Find seeds" heading regardless of whether any are available, and
say there are none if there aren't any, along with a link to add some
- moved "sign in or sign up to add seeds" to under the Find seeds area
Merge pull request #260 from Skud/seed-tweaks

Minor tweaks to seed trading: