Make Twitter-share-link use correct base URL/path.

Make the "Share on Twitter"-link use the correct "base"-URL cf. "Tweet Button | Twitter Developers" (https://dev.twitter.com/docs/tweet-button) plus use "text"-URL-param. I.e. use "https://twitter.com/share?text=<default-Tweet-text>" instead of "https://twitter.com/?status=<default-Tweet-text>".