Use acts_as_taggable for investment valuation tags

We were manually doing the same thing, generating inconsistent results,
since the method `valuation_tag_list` was using the `valuation` context,
when actually the expected behavior would be to use the `valuation_tag`
context.

app/views/admin/budget_investments/show.html.erb

@@ -29,7 +29,7 @@
 <p id="tags">
   <strong><%= t("admin.budget_investments.show.tags") %>:</strong>
 
-  <%= @investment.tags_on(:valuation).pluck(:name).sort.join(", ") %>
+  <%= @investment.valuation_tags.pluck(:name).sort.join(", ") %>
 </p>
 
 <p id="assigned_valuator_groups">