Feature: View a contact's previous conversation (#422)

* Add API to fetch conversations of a contact

* Add conversation list in sidebar

app/controllers/api/v1/contacts/conversations_controller.rb

@@ -0,0 +1,23 @@
+class Api::V1::Contacts::ConversationsController < Api::BaseController
+  def index
+    @conversations = current_account.conversations.includes(
+      :assignee, :contact, :inbox
+    ).where(inbox_id: inbox_ids, contact_id: permitted_params[:contact_id])
+  end
+
+  private
+
+  def inbox_ids
+    if current_user.administrator?
+      current_account.inboxes.pluck(:id)
+    elsif current_user.agent?
+      current_user.assigned_inboxes.pluck(:id)
+    else
+      []
+    end
+  end
+
+  def permitted_params
+    params.permit(:contact_id)
+  end
+end