Fix Admin interface with sub directories

This PR fixes most of the admin interface to work with a subdirectory configured.

Issues left for full subdir support:
 - Cleanup paths in `core/server/`
 - Image upload is borked (localStorage returns wrong path)
 - Blog Cover/User Cover don't show properly (client-side template)

I'm going to address the remaining issues today. Since this PR adds the asset helper that @bastilian is working on I wanted to get this merged first.