feat: Assignable agents API (#2131)

`inboxes/id/assignable_agents` returns all users that have permission to a conversation happening in that inbox.

app/policies/inbox_policy.rb

@@ -23,6 +23,10 @@ class InboxPolicy < ApplicationPolicy
     true
   end
 
+  def assignable_agents?
+    true
+  end
+
   def create?
     @account_user.administrator?
   end