Manage access to non-existent animals

[splitbrain]

It should be configurable what happens when a non-existent animal is accessed:

• show a list of existing animals
• show a 404 error
• redirect to configured URL
• show the farmer (current solution)

[splitbrain]

Currently only a single hostname can be configured for the farmer. Multiple should be possible.

[splitbrain]

There is currently no good way to generate the URL of an animal from its name. That makes linking to the animal complicated/impossible.