Only set 'url' if permalink in metadata and remove duplicate confirm msg

Only set 'url' if permalink in metadata and remove duplicate confirm msg

The current behaviour addes a `url` attribute to the frontmatter of all posts imported from Jeklly and assumes the desired permalink structure is /:year/:month/:day/:title/. This may be the case for most peeps, but poses a problem for those that don't use this permalink structure as the `url` attribute takes precedence over the `permalink` attribute in the site-wide configuration meaning it can't be overruled.

This changes the behaviour to only set the `url` attribute if the `permalink` attribute is set in the Jekyll frontmatter.

The duplication of the confirmation message is also removed.

Tests have been updated to reflect this change in behaviour.

Fixes #1887