Add notice when supporting an investment

It was hard to notice what was going on when supporting one investment
which was at the bottom of the investment index page.

I wonder whether we should add the title of the investment to this text;
I'm not doing so because we don't do that anywhere else.

spec/system/management/budget_investments_spec.rb

@@ -329,6 +329,7 @@ describe "Budget Investments" do
 
       expect(page).to have_content "1 support"
       expect(page).to have_content "You have already supported this investment project. Share it!"
+      expect(page).to have_content "Investment supported successfully"
       expect(page).to have_content "CONSUL\nMANAGEMENT"
     end