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

[srid]

For #494

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