Update permissions.

Protect updating a feed with the "feedfeeder: Update feed" permission.

Protect updating all feeds in a mega update with the "feedfeeder:
Update all feeds" permission.

We give these to the Manager and Site Administrator roles in an
upgrade step.

Fixes https://github.com/collective/Products.feedfeeder/issues/4
Take title as basis for uid of item if guid and link are not found.

They are optional in rss.