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.

lib/tasks/migrations.rake

@@ -0,0 +1,7 @@
+namespace :migrations do
+  desc "Migrates context of valuation taggings"
+  task valuation_taggings: :environment do
+    ApplicationLogger.new.info "Updating valuation taggings context"
+    Tagging.where(context: "valuation").update_all(context: "valuation_tags")
+  end
+end