Add index_path to allow arbitrary page used for index

[preaction]

Rather than forcing the user to pick an index app, which then has complete control over what the index page really is, allow an index_path that allows for a full path.

Do not allow index and index_path at the same time. Use index_path for the index going forward. We're going to deprecate index.

[preaction]

We'll solve this by making index dual-purpose for now and deprecate the app version.