Improved twitter username function for #270

When users edit their or others' Twitter URL in the admin panel, the Twitter url `https://twitter.com/username` or Twitter handle `@username` is cut down to `username`. Things that are not a valid Twitter handle (characters not in the set `[a-zA-Z0-9_]` will cause an error.

`largo_twitter_url_to_username` now works for `@username`, trailing slashes and URL parameters. It will let through invalid characters in a username, in the unlikely case that the reader has pasted in `https://facebook.com/user.name` at some point in the past.

The author bio social media template was updated to reflect these changes. It was the only place that used the author Twitter URL as a URL without parsing it for the `username`.