website/camweb: redirect to right scheme on www. request

Fixes http://camlistore.org/459

Change-Id: Icf69e404a6985c984688c291e88da365721ee029
importers: add SupportsIncremental accessor

If an importer is efficient at running regularly, it returns true.

If it's not true, the UI won't allow turning on automatic runs.

Flickr doesn't set it (yet).