Remove incorrect use of `p-name` on non-articles

As raised by aaronpk on the IndieWeb-dev chat, using the p-name here is not correct, as it should be only used for the title of a post. In these cases, these content types do not have a title (as that is reserved for articles), just content.

This will screw with post-type-discovery, as that will return article when it finds a p-name.

In the case of RSVPs, we can remove the p-name completely, as it was meant to be hidden metadata, which now no longer makes sense.

Closes #894 (closed).