Prevent adding dupes across years

[ebidel]

Dupes are prevented by checking if the URL has already been posted. But we
only look a the current year. IOW, an RSS post from 2017 can be reposted via the extension
in 2018 because we only consider the submitted year:

https://github.com/ebidel/devwebfeed/blob/62d9c697f8827b227bba621ae21a001ed49aeb49/public/firebaseHelper.mjs#L41-L59