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

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

app/views/api/v1/accounts/inboxes/assignable_agents.json.jbuilder

@@ -0,0 +1,5 @@
+json.payload do
+  json.array! @assignable_agents do |agent|
+    json.partial! 'api/v1/models/agent.json.jbuilder', resource: agent
+  end
+end