feat: Assignable agents API (#2131)

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

app/controllers/api/v1/accounts/inboxes_controller.rb

@@ -7,6 +7,10 @@ class Api::V1::Accounts::InboxesController < Api::V1::Accounts::BaseController
     @inboxes = policy_scope(Current.account.inboxes.order_by_name.includes(:channel, { avatar_attachment: [:blob] }))
   end
 
+  def assignable_agents
+    @assignable_agents = (Current.account.users.where(id: @inbox.members.select(:user_id)) + Current.account.administrators).uniq
+  end
+
   def create
     ActiveRecord::Base.transaction do
       channel = create_channel