Extract management_budget_investment from shared nested documentable spec to system specs

Replaced 'login_as' with 'do_login_for' using 'management: management_section?' to
handle login requirements correctly for each context.

Also removed the now-unused 'documentable_fill_new_valid_budget_investment' helper
from common actions.

Note that it does not seem necessary to create an administrator with the user, as was
done in the original shared example. Also, as in the previous commit, it appears that
we do not need to set the user as the author when creating the documentable.

While reviewing this, we also noticed that the create(:administrator, user: user) call
was unnecessarily included in the nested_imageable system spec in commit cdfaec5217 when
the path is a management section. So we use this commit to remove the unnecessary condition.

spec/system/nested_imageable_spec.rb

@@ -52,7 +52,7 @@ describe "Nested imageable" do
 
   context "New and edit path" do
     before do
-      create(:administrator, user: user) if admin_section? || management_section?
+      create(:administrator, user: user) if admin_section?
       imageable.update!(author: user) if edit_path?
       do_login_for(user, management: management_section?)
       visit path