Fix Admin interface with sub directories

[halfdan]

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.

[ErisDS]

So in terms of cleaning up paths, one of the problems we have is that the base path or subdir is often '/'. We should not use path.join for URLs because this produces the incorrect result on Windows.

We need to ensure there is consistency - does the subdir always end with a slash even if it is just slash?

If so, we either add a non-slashed version, which is ugly, or we can use url.resolve() instead of path.join.

[halfdan]

@ErisDS, I don't recall using a path.join in the PR? After this PR I wanted to clean up paths by writing a new utils module with a helper that will handle these issues, but I guess we could enforce the trailing slash and then use url.resolve.

[ErisDS]

No it's not in this PR, I was responding to your todo list just while I thought of it.