Ask for confirmation when hiding/blocking users

In the moderation section there's no clear indicator as to what the
"Hide" and "Block" buttons do and the difference between them.

Since we're using confirmation dialogs in all moderation actions except
these ones, we're adding them here as well, so the difference will
appear in the dialog.

This isn't a very good solution, though, since the confirmation dialog
comes after clicking the button and users have already been wondering
whether clicking that button will be the right choice. A better solution
would be making the purpose clear before the button is clicked, although
that's something we don't do anywhere in the admin/moderation sections.

spec/system/moderation/users_spec.rb

@@ -24,7 +24,9 @@ describe "Moderate users" do
     visit debate_path(debate1)
 
     within("#debate_#{debate1.id}") do
-      accept_confirm("Are you sure? Block author \"#{debate1.author.name}\"") { click_button "Block author" }
+      accept_confirm("Are you sure? This will hide the user \"#{debate1.author.name}\" and all their contents.") do
+        click_button "Block author"
+      end
     end
 
     expect(page).to have_current_path(debates_path)
@@ -64,7 +66,8 @@ describe "Moderate users" do
     within("#moderation_users") do
       expect(page).to have_content citizen.name
       expect(page).not_to have_content "Blocked"
-      click_button "Block"
+
+      accept_confirm { click_button "Block" }
     end
 
     within("#moderation_users") do