"add post" functionality

[Robert Jäschke]

It would be helpful (e.g., for automation) to have a mechanism that let's BibSonomy automatically decide, if a posted URL should be stored as a bookmark or as a publication.

1. When a URL is submitted to /addPost (or /postPost) BibSonomy checks, if the URL can be scraped.
2. If it can be scraped, the controller for handling publications proceeds and presents the user the publication edit form.
3. Otherwise, the controller for handling bookmarks proceeds.
4. For that to work, we need to ensure that our scrapers only return positive results for valid pages, which is currently not the case. See #2102.

[Robert Jäschke]

let's do it

[Robert Jäschke]

When the post is automatically posted as a publication, the user should have the option to post it as a bookmark.

[Robert Jäschke]

Issue #2550 was marked as a duplicate of this issue.