Fix volunteer counts shown for a team (#46)

Before this change the count of volunteers in a team was not in sync with the filters applied.

![image](/attachments/0a3a0c5c-f3c2-4ef4-89d7-0bcf1fb9302d)

After the change:
![image](/attachments/b88719f6-fadf-4f07-b4d4-985533b0ea2f)

BTW I feel that we will end up refactoring the relation at some point in the future.

Reviewed-on: #46
Co-authored-by: Tocho Tochev <tocho@tochev.net>
Co-committed-by: Tocho Tochev <tocho@tochev.net>

app/models/volunteer_team.rb

@@ -1,6 +1,7 @@
 class VolunteerTeam < ActiveRecord::Base
   belongs_to :conference
-  has_and_belongs_to_many :volunteers
+  has_many :volunteers, inverse_of: :volunteer_team
+  has_and_belongs_to_many :supporters, class_name: "Volunteer", inverse_of: :additional_volunteer_teams
 
   validates :name, presence: true
   validates :color, presence: true, format: {with: /\A#?[a-f0-9]{6}\z/i}