Some small fixes

[rimas-kudelis]

I think these are small enough to be in a single PR, at least in this case. The changes proposed here are:

• using LEFT JOIN in SELECT queries instead of WHERE. This allows exporting articles with missing authors and/or categories
• JoomlaContent::isPublished() now only returns true if the article was marked as Published in Joomla. Previously it would return true for Unpublished articles as well, but not for Archived or Trashed articles, and was used to filter out which articles not to export at all. After this update, all articles will be exported, but those not currently Published in Joomla will be marked as draft: true. Perhaps the filtering logic could be reintroduced, but use some new property.
• the created date of Joomla articles is now exported as date, and modified date as lastmod (previously, only modified date was exported as date). I myself actually used the publish_up value as date field, but that should probably be exported into the publishDate field instead. I didn't try that scenario though, so leaving this nuance as-is.
• two seemingly unused *.toml.ftl templates have been removed.

I didn't even look at tests, so it's possible that some are now failing.

[rimas-kudelis]

@davetcc, care to review? I'd like to delete my fork already.

[davetcc]

Apologies, I had not noticed this, I will review immediately.

[davetcc]

All good, thanks for raising the PR, and apologies for the delay in reviewing.

[rimas-kudelis]

Thank you! :)