EthicalAds: inject ad only if it's a known theme and above the fold (#…

…329)

Disable using `stickybox` style for now and only inject the ad if we are
sure it's a known theme and the ad it's going to be above the fold.

In a following iteration, we will want to use `stickybox` in the other
cases but we want to have the ability to disable the re-placement in
smaller screens.

Closes #328

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Co-authored-by: Eric Holscher <[email protected]>