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.

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)

Avoid extra comparison function.

Use WordPress function `wp_registration_url` for fetching registration URL while still keeping compatibility with older WordPress (bellow 3.6) that do not have this function.

Filtered due to lack of support in MySQL < 5.5.3.

From PRs by @pupi1985 (unmergeable due to file structure changes).
Closes #69.
Closes #126.