Better user agent string, suppress flooding of the community page by single users

* The user agent string that was used within friendica inconsistent and was partly a user agent string from a regular browser (what shouldn't be done). This ist now changed to "Mozilla/5.0 (compatible; Friendica)"
* Sometimes you have users on your system that are flooding the community page. There is now a configuration value in the .htconfig.php ($a->config['system']['max_author_posts_community_page']) to define a maximum count of items per person.