Allow full path in "index" config

[preaction]

With the new index_path (#325), users can make a full path the index. Rename the current index to index_app to let the app choose what the index should be.

Though, perhaps we shouldn't allow that? Perhaps index_path is the only thing we need?

[preaction]

Nah, leading slash makes index path. Deprecate the app version, that makes 'pages' less magic which is good. Make '/index.html' optional.