Add filters on budget investments index page

Now it's easier to change the investments filter. Previously we had to
go back to the budget index page, change the filter there, and then
select one heading.

Now the links to change the current filter in the budget index page
aren't needed anymore.

app/helpers/budgets_helper.rb

@@ -1,8 +1,4 @@
 module BudgetsHelper
-  def show_links_to_budget_investments(budget)
-    ["balloting", "reviewing_ballots", "finished"].include? budget.phase
-  end
-
   def budget_voting_styles_select_options
     Budget::VOTING_STYLES.map do |style|
       [Budget.human_attribute_name("voting_style_#{style}"), style]