Feed unhiding automatic when user adds it to a Fandom

Whether a feed is “hidden” is a user-feed relationship, separate from the feed being on/off fandoms. This led to the possibility that a user has hidden a feed in the past, but now is interested in it and adds it to a fandom (and is surprised to see that it lands on the fandom in a hidden state). Because the typical user intention is to see feeds added to Fandoms, I am adding the functionality of automatically unhiding feeds as to a user when that user adds the to a fandom.

This should avoid some cases of user confusion.