Use "resolve" for polymorphic hierarchy paths

In the past, we couldn't use `polymorphic_path` in many places. For
instance, `polymorphic_path(budget, investment)` would return
`budget_budget_investment_path`, while in our routes we had defined
`budget_investment_path`.

With the `resolve` method, introduced in Rails 5.1, we can use symbols
to define we want it to use `investment` instead of `budget_investment`.
It also works with nested resources, so now we can write
`polymorphic_path(investment)`.

This makes the code for `resource_hierarchy_for` almost impossible to
understand. I reached this result after having a look at the internals
of the `resolve` method in order to get its results and then remove the
symbols we include.

Note using this method will not make admin routes compatible with
`polymorphic_path`. Quoting from the Rails documentation:

> This custom behavior only applies to simple polymorphic URLs where a
> single model instance is passed and not more complicated forms, e.g:
> [example showing admin routes won't work]

Also note that now the `admin_polymorphic_path` method will not work for
every model due to inconsistencies in our admin routes. For instance, we
define `groups` and `budget_investments`; we should either use the
`budget_` prefix in all places or remove it everywhere. Right now the
code only works for items with the prefix; it isn't a big deal because
we never call it with an item without the prefix.

Finally, for unknown reasons some routing tests fail if we use
`polymorphic_path`, so we need to redefine that method in those tests
and force the `only_path: true` option.

config/routes/admin.rb

@@ -253,3 +253,15 @@ namespace :admin do
     resources :imports, only: [:new, :create, :show]
   end
 end
+
+resolve "Milestone" do |milestone|
+  [*resource_hierarchy_for(milestone.milestoneable), milestone]
+end
+
+resolve "ProgressBar" do |progress_bar|
+  [*resource_hierarchy_for(progress_bar.progressable), progress_bar]
+end
+
+resolve "Audit" do |audit|
+  [*resource_hierarchy_for(audit.associated || audit.auditable), audit]
+end