Fix unreachable posts with slug "meta" or "edit" multi-user mode

In multi-user mode the draft-editing routes /{action}/edit and /{action}/meta
would always take precedence over any collection post whose slug was "edit" or
"meta", so /myblog/meta rendered "This page is missing" instead of the post.

This moves draft editing to the /d/ path in both single- and multi-user mode,
and normalizes the metadata path to /d/{action}/edit/meta to match the
collection editing path, while keeping post *viewing* URLs unchanged.

This also fixes a dead edit link in post.tmpl that pointed at /edit/{id},
for which no route exists.

Fixes #278