Increase the number of custom placement slots for campaign prompts

[aschweigert]

...expand their recommended use, and make them usable in more places

Asana: https://app.asana.com/0/1200942827698993/1201001433518046/f
Details: https://docs.google.com/document/d/18dBs-2PiYre8msDaZswRdciSmqExVfMrN9GgpaO9qsg/edit#heading=h.4i9yh2ae3yos

This is mostly about being able to measure the performance of CTAs consistently.

I agree that the limit of 3 custom placements feels arbitrary, but some of these use-cases (for example: using the newsletter signup prompt both inline on article pages and as the primary signup form on a newsletter signup page) might be out of scope for this particular plugin.

Removing or increasing that limitation feels like a good first step but this will require a bit more thought to work through the details of what we want to support (or not).