Newznab cleanup

Improved the detection which means usenet-crawler should work now, on the same pages that the actual newznab does.

There are other areas this can be improved though, but I don't have access to many newznab services.

Page injection has been improved slightly. We are still importing to every unknown page (I can see a new option coming along I think), as well as excluding enabled newznab pages, but we not don't import to ignored pages.

The usenet-crawler integration has however highlighted my ever poor design skills. CSS from the parent page gives for a very poor experience, with different font sizes and colours, rendering this push potentially "not fit for purpose".