Volunteer can edit foster's dog

[maggie-larkin]

Notes

• A foster could be fostering multiple dogs, so we'd probably want a mutli-select dropdown like: https://harvesthq.github.io/chosen/
• See here for mock

User Story

As a volunteer
I want to assign this foster an animal
So that I can keep track of where the dogs are placed

Acceptance Criteria

GIVEN I am a volunteer 
AND I am on an individual foster's page

THEN I see a dropdown field that has the label "Animal" 
AND the options are all the dogs that don't have the status "adopted" yet
AND the options are listed in alphabetical order
AND I can select multiple dogs