Add option to obfuscate domain name in public list of domain blocks (#15355)

- Replace the middle of the domain with * characters (except for periods)
- Add SHA-256 digest of the domain name in tooltip

db/migrate/20201218054746_add_obfuscate_to_domain_blocks.rb

@@ -0,0 +1,15 @@
+require Rails.root.join('lib', 'mastodon', 'migration_helpers')
+
+class AddObfuscateToDomainBlocks < ActiveRecord::Migration[5.2]
+  include Mastodon::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  def up
+    safety_assured { add_column_with_default :domain_blocks, :obfuscate, :boolean, default: false, allow_null: false }
+  end
+
+  def down
+    remove_column :domain_blocks, :obfuscate
+  end
+end