Add config option for primary site `/blog` prefix on posts permastructure

Some sites need to use the `/blog` prefix that WP adds by default to post permalinks however we remove it currently to allow flexibility.

The blog prefix is notably desirable when using a page for posts withe slug `blog`.

We have a few options that would help to improve this:

- check the page for posts and it's slug, if it's blog then don't filter the permastructure option
- add a config option for whether to remove the `/blog` prefix, defaulting to removing it