Add option to remove substack nags

[Pycea]

Despite apparent promises to the contrary, substack still shows various nags to subscribe or upgrade your subscription.

The only reason this is an option instead of on by default is that if it is on, there isn't an obvious way to actually subscribe if you want to (there's "Manage subscription" under the drop down menu, but that's kind of hidden). Maybe a compromise would be to always hide the in-post nags, and have the option to hide the top bar buttons?

Tested for subscriber, non-subscriber, and anonymous user.

[maksverver]

Do you want to resolve the conflicts or should I do it? (I don't know which option order you prefer.)

[Pycea]

I can resolve, give me two mins.

[Pycea]

Should be good to go now.