Simplify action links in budgets table

The word "budget" in the "Preview budget" link is redundant.

On the other hand, the words "Manage", Edit" and "Admin" are not
really necessary in my humble opinion. Just like in the admin
navigation menu we use "Participatory budgets" instead of "Manage
Participatory budgets", the fact that we're going to manage or
admin or edit something can be deduced from the fact that we're in
the admin section.

Besides, it isn't clear to me why we use "Manage" for projects,
"Edit" for heading groups and "Admin" for ballots. The differences
between these three concepts might be too subtle for me.

The previous paragraphs haven't been corroborated with real users,
though, so I might be mistaken and we might need to revisit these
links in the future.

These actions still take quite a lot of space. Maybe in the future we
could remove the "delete" icon, at least on budgets which cannot be
deleted.

spec/system/admin/budgets_spec.rb

@@ -116,7 +116,7 @@ describe "Admin budgets", :admin do
     scenario "Can preview budget before it is published" do
       visit edit_admin_budget_path(budget)
 
-      within_window(window_opened_by { click_link "Preview budget" }) do
+      within_window(window_opened_by { click_link "Preview" }) do
         expect(page).to have_current_path budget_path(budget)
       end
     end
@@ -130,7 +130,7 @@ describe "Admin budgets", :admin do
       expect(page).not_to have_content "This participatory budget is in draft mode"
       expect(page).not_to have_link "Publish budget"
 
-      within_window(window_opened_by { click_link "Preview budget" }) do
+      within_window(window_opened_by { click_link "Preview" }) do
         expect(page).to have_current_path budget_path(budget)
       end
     end