block_rss_client: MDL-13932 Clean up a lot of the crap.
This was all in order to do MDL-19889! * The old tabbed interface is gone. * The manage feeds UI has been completely rewritten to be a few separate sane scripts like managefeeds.php, editfeed.php and viewfeed.php * This is used from both Admin -> plugins -> blocks ... and as a link from the block edit_form.php * new edit_form.php for the block replacing config_istance.php. * A lot of stuff to do with UI specific to this block has been removed from rsslib.php