[Reader] Use "follow" for tags

[thomashorta]

We recently changed several Strings in the apps from "follow"/"followed"/"following" to "subscribe"/"subscribed"/"subscriptions", but it turns out these new terms should be only used for blogs, so we now need to revert some of those changes back to the "follow" when talking about tags.

Internal ref: p1709029253585639/1708546908.187529-slack-C05N140C8H5

[dangermattic]

Thanks for reporting! 👍