Also merge Joomla helper into external users file.

We cannot get the id/username from the display name, so we have to
display usernames instead.

Also use $definitions variable in queries.

And now we have local Joomla integration, we don't have to fail if the Joomla plugin isn't installed.

Added local Joomla integration rather than relying on Joomla having the separate plugin installed. (But still support the plugin if it's there, as it gives a lot more flexibility)

For logged-out users only.

Set up CacheManager as a Singleton.

Fixes #387.