fx30: use isValidFeed in Feeds.jsm

* https://bugzilla.mozilla.org/show_bug.cgi?id=961529
remove unnecessary parameters
fix wrong function name
use mediaURL instead of obsolete imageURL

* https://bugzilla.mozilla.org/show_bug.cgi?id=497098
use window.Services
ignore clicks except left button to avoid unexpected action
minor fix

* update comments