fbpost/fbsync: fetching larger photos, appnet/statusnet/twitter: better removal of superfluous links

* Facebook doesn't make it easy to fetch pictures in a high resolution. Hopefully these changes will improve it.
* If link information was added to content from appnet/statusnet/twitter, the link can be removed under certain circumstances (to improve the design of the post) - this behaviour is now improved.
* fbpost/gpluspost: When posting shared posts, a link to the original post is now added at the bottom of the post.