Addressing index with / creates a ghost document

Open https://da.live/edit#/owner/repo/: you start editing a page named `repo` - which is wrong - you should be editing `/index` of the project (that's true with nested folders)

This has also a weird internal effect: in R2, it creates an extension less `repo` document under `/owner`. Then, version history is broken.

Expected behavior: opening https://da.live/edit#/owner/repo/ should take you to https://da.live/edit#/owner/repo/index.

cc @tripodsan

Start by reproducing the URL entry point at https://da.live/edit#/owner/repo/ and compare it with the expected /index URL. Trace the address handling for root and nested folders, then verify that opening a trailing-slash project URL selects /index, does not create a ghost extensionless document, and preserves version history.

Written by the indexing model from the issue text.