The javascript that prompts for the mastodon instance should strip duplicated 'https://' protocol in user response

In the new prompt for a mastodon instance, users can enter anything and then that gets added to the URL in the mastodon-share element.

Prompted for an instance, it is quite possible that users will enter a full URL. So in order to avoid the result in the mastodon-share element being weird, we should strip the 'https://' prefix from the instance variable after the prompt returns, if that prefix is present.