Allow user to specify target page id for posts

Instead of adding the ability to specify pages, we can just document in the UI that the module only supports authorizing access to a single page. This will keep things simpler for now. Users will just have to make sure to only select a single page during the authorization process.

This has actually turned into a somewhat larger refactor than I had initially anticipated, but it couldn't really be helped. Posting methods are being moved out of the network plugin and into the FacebookPostAuthManager. I have managed to get posting working in the issue fork. I think the only remaining step here is to make sure to sufficiently document for the end user that they should only authorize posting to a single page.

As for the documentation, it will be important that if someone accidentally authorizes posting to multiple pages, they will need to delete the connection to their account on the Drupal side, and re-add it. During authorization, they should click the link, "Edit previous settings", which will allow them to update their selection of pages that the app can post to.

I actually set it up so that if someone authorizes posting to more than one page, then the account details will not be added to Drupal.