Use a button instead of a link to block users

We're continuing to replace links with buttons, for the reasons
explained in commit 5311daadf.

We're also adding an ARIA label since on the same page there might be
several links to block different users.

spec/system/moderation/budget_investments_spec.rb

@@ -75,7 +75,7 @@ describe "Moderate budget investments" do
           click_button "Search"
 
           within "tr", text: investment.author.name do
-            expect(page).to have_link "Block"
+            expect(page).to have_button "Block"
           end
         end