Simplepie will try quite aggresively to autodiscover rss feeds by
default. Make this configurable when adding a feed and also store
the autodiscovered location rather than the input location.
I also removed the feed validator as this doesn't make much sense with
feed autodiscovery, and I think we can improve simplepie integration to
do this instead
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
