Fix wrongful matching of last period in extended usernames

Fix anchor tags in some wikipedia URLs being matches as a hashtag

spec/models/account_spec.rb

@@ -178,7 +178,6 @@ RSpec.describe Account, type: :model do
     end
   end
 
-
   describe 'MENTION_RE' do
     subject { Account::MENTION_RE }
 
@@ -190,6 +189,14 @@ RSpec.describe Account, type: :model do
       expect(subject.match('@alice Hey how are you?')[1]).to eq 'alice'
     end
 
+    it 'matches full usernames' do
+      expect(subject.match('@alice@example.com')[1]).to eq 'alice@example.com'
+    end
+
+    it 'matches full usernames with a dot at the end' do
+      expect(subject.match('Hello @alice@example.com.')[1]).to eq 'alice@example.com'
+    end
+
     it 'matches dot-prepended usernames' do
       expect(subject.match('.@alice I want everybody to see this')[1]).to eq 'alice'
     end