Move phases and actions out of the budget form

Having links in the middle of a form distracts users from the task of
filling in the form, and following a link before submitting the form
will mean whatever has been filled in is lost.

And the budgets form is already very long and hard to fill in. Having
the phases table in the middle of it made it even harder. And, since
we're planning to add the option to manage groups and headings from the
same page, it's better to have a dedicated page for the form.

spec/system/admin/budget_phases_spec.rb

@@ -13,7 +13,7 @@ describe "Admin budget phases" do
       uncheck "budget_phase_enabled"
       click_button "Save changes"
 
-      expect(page).to have_current_path(edit_admin_budget_path(budget))
+      expect(page).to have_current_path(admin_budget_path(budget))
       expect(page).to have_content "Changes saved"
 
       expect(budget.current_phase.starts_at.to_date).to eq((Date.current + 1.day).to_date)
@@ -23,7 +23,7 @@ describe "Admin budget phases" do
     end
 
     scenario "Show default phase name or custom if present" do
-      visit edit_admin_budget_path(budget)
+      visit admin_budget_path(budget)
 
       within_table "Phases" do
         expect(page).to have_content "Accepting projects"