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.

spec/lib/tasks/migrations_spec.rb

@@ -0,0 +1,22 @@
+require "rails_helper"
+
+describe "Migration tasks" do
+  let(:run_rake_task) do
+    Rake::Task["migrations:valuation_taggings"].reenable
+    Rake.application.invoke_task("migrations:valuation_taggings")
+  end
+
+  it "updates taggings" do
+    valuation_tagging = create(:tagging, context: "valuation")
+    another_valuation_tagging = create(:tagging, context: "valuation")
+    valuation_tags_tagging = create(:tagging, context: "valuation_tags")
+    tags_tagging = create(:tagging)
+
+    run_rake_task
+
+    expect(valuation_tagging.reload.context).to eq "valuation_tags"
+    expect(another_valuation_tagging.reload.context).to eq "valuation_tags"
+    expect(valuation_tags_tagging.reload.context).to eq "valuation_tags"
+    expect(tags_tagging.reload.context).to eq "tags"
+  end
+end