Add support for fb:// urls

Some pages on the mobile site now link to fb:// urls. It would be nice to rewrite these to http urls.

Currently, the "See your memories" link points to fb://onthisday/?source=shared_feed_story and clicking on it displays this message:

Web page not available The web page at fb://onthisday/?source=shared_feed_story could not be loaded because: net::ERR_UNKNOWN_URL_SCHEME

We should be able to rewrite that to https://m.facebook.com/onthisday/?source=shared_feed_story

I'll try and throw together a pull request for this in the near future.

I'll try and throw together a pull request for this in the near future.

No longer a need for this. Facebook fixed whatever they changed.