Allow blocking TLDs, and fix TLD blocks not being editable (#12805)

Fixes #12795

It was already possible to create domain blocks for TLDs, but those
weren't enforced, nor editable. This commit changes it so that they
are enforced and editable.

spec/models/domain_block_spec.rb

@@ -52,6 +52,16 @@ RSpec.describe DomainBlock, type: :model do
       block = Fabricate(:domain_block, domain: 'sub.example.com')
       expect(DomainBlock.rule_for('sub.example.com')).to eq block
     end
+
+    it 'returns a rule matching a blocked TLD' do
+      block = Fabricate(:domain_block, domain: 'google')
+      expect(DomainBlock.rule_for('google')).to eq block
+    end
+
+    it 'returns a rule matching a subdomain of a blocked TLD' do
+      block = Fabricate(:domain_block, domain: 'google')
+      expect(DomainBlock.rule_for('maps.google')).to eq block
+    end
   end
 
   describe '#stricter_than?' do