Jetpack Breaks Frontenberg By Loading wp-admin/includes/post.php

#### Steps to reproduce the issue

1. Install Frontenberg and Jetpack
2. Access any URL that has more than one level e.g. `/foo/bar`
3. HTTP 500

#### What I expected

For Jetpack to not influence the page

#### What happened instead

```
Sentry\Exception\FatalErrorException: Error: Cannot redeclare wp_check_post_lock() (previously declared in /srv/www/tomjn.com/htdocs/wp-content/themes/Frontenberg/includes/shims.php:84)
#0 /wp-admin/includes/post.php(1592): null
```

Because Jetpack loaded it here:

https://github.com/Automattic/jetpack/blob/master/projects/plugins/jetpack/sal/class.json-api-post-base.php#L12

---

`SAL_Post` might need this file, but `SAL_Post` isn't always needed, **_but it's loaded anyway_**, as is `wp-admin/includes.post.php`.

At this point I can't load that file myself to avoid the problem as it then triggers a large chain of dependent files. Eitherway this is a frontend UI that has nothing to do with `SAL_Post` and its child classes. Unless this gets fixed I'm going to have to write an option filter to strip Jetpack out of the active plugin list when Frontenberg runs :/ Removing the shim breaks Frontenberg too