Identity: Add Layout Scaffolding

### Is your feature request related to a problem? Please describe.

I want my login, registration, etc. pages to use a different `_Layout.cshtml` than my "authenticated" layout.

As such, I need to scaffold out **every single page in Identity** to signify a different `Layout` property value.

### Describe the solution you'd like

In the current Identity scaffolding, I can scaffold `Manage\Layout`, but that only applies to the "manage" pages.

I'd like to see other `Layout` scaffolds that can apply to the remainder of the pages, especially the ones that apply to unauthenticated users.