Be strict about naming for Folder (only for in-db dashboard folders) vs Directory (for actual filesystem)

[fm3]

There are a couple of spots where we use the terms interchangeably (at least folder for filesystem stuff). Not a huge issue, but I think it would be nice to use the words strictly as in the issue title, in order to immediately know what to look for.

This also concerns the config options datastore.baseFolder and datastore.localFolderWhitelist (config migration will be needed. also referenced in messages file)