app.net, facebook, twitter and statusnet: Preview now shows the post like it will appear on other networks/Better contact handling

In the connectors for app.net, facebook, twitter and statusnet there is now a better contact handling when importing the timeline of these networks. In the past there were several cases where contacts were added by mistake.

Additionally (and maybe more important): When clicking on "preview" the preview is now generated like it will appear on these networks. This helps with checking the posts lengths.

**Important!**
This pull request mustn't be accepted before the pull request for the core was accepted.