Added support for configurable reserved usernames (fix of #1382) (#3566)

* Added support for configurable reserved usernames

* Added reserved usernames from mastodon issue 1355

* Fix reserved usernames

spec/models/account_spec.rb

@@ -381,6 +381,12 @@ RSpec.describe Account, type: :model do
       expect(account_2).to model_have_error_on_field(:username)
     end
 
+    it 'is invalid if the username is reserved' do
+      account = Fabricate.build(:account, username: 'support')
+      account.valid?
+      expect(account).to model_have_error_on_field(:username)
+    end
+
     context 'when is local' do
       it 'is invalid if the username doesn\'t only contains letters, numbers and underscores' do
         account = Fabricate.build(:account, username: 'the-doctor')