Folder notes can now live in $folder/index.md

[srid]

For #494

• foo/index.md should be equivalent to foo.md
• foo/index.yaml should be equivalent to foo.yaml
• How to deal with broken edit button link for index.md notes?
• Refactor

[srid]

• How to deal with broken edit button link for index.md notes?

Because, the source-path is now no longer accurate:

We can't use encodeRoute anymore - as the information about the index.md slug is lost.