In general, we always use relative URLs (using `_path`), but sometimes
we were accidentally using absolute URLs (using `_url`). It's been
reported i might cause some isuses if accepting both HTTP and HTTPS
connections, although we've never seen the case.
In any case, this change makes the code more consistent and makes the
generated HTML cleaner.
The path to investments in lists is generated with a helper
method named namespaced_budget_investments_path. This allows
its use in the shared suggest partial. Also, as nested
resource in routes, link_to("title", investments) fails.
