与MVC5中现有用户数据库的复杂身份验证

I'm migrating a SaaS app from Classic ASP to .NET MVC5 and will use EF6 Database First. The login form for end users is customisable by each tenant (on their own subdomain but pointing to the same web application). We wish to use the existing database schema and the new authentication & authorization filters. I'm migrating a SaaS app from Classic ASP to .N