Require paths to be absolute when adding a storage folder

Created by: VoidingWarranties

When writing the API docs for the storage endpoints I realized it might be confusing to add a relative path using the API, as the path would be relative to siad. This is especially confusing if you then restart siad in a different directory.

Another option is to convert the path to an absolute path, so that it's full path is shown in the list of storage folders.