Allow hyphens in the middle of remote user names (#9345)

Fixes #9309

This only allows hyphens in the middle of a username, much like dots,
although I don't have a compelling reason to do so other than keeping
the changes minimal.

app/models/account.rb

@@ -49,7 +49,7 @@
 #
 
 class Account < ApplicationRecord
-  USERNAME_RE = /[a-z0-9_]+([a-z0-9_\.]+[a-z0-9_]+)?/i
+  USERNAME_RE = /[a-z0-9_]+([a-z0-9_\.-]+[a-z0-9_]+)?/i
   MENTION_RE  = /(?<=^|[^\/[:word:]])@((#{USERNAME_RE})(?:@[a-z0-9\.\-]+[a-z0-9]+)?)/i
 
   include AccountAvatar

spec/models/account_spec.rb

@@ -618,9 +618,15 @@ RSpec.describe Account, type: :model do
         expect(account).not_to model_have_error_on_field(:username)
       end
 
-      it 'is invalid if the username doesn\'t only contains letters, numbers and underscores' do
+      it 'is valid even if the username contains hyphens' do
         account = Fabricate.build(:account, domain: 'domain', username: 'the-doctor')
         account.valid?
+        expect(account).to_not model_have_error_on_field(:username)
+      end
+
+      it 'is invalid if the username doesn\'t only contains letters, numbers, underscores and hyphens' do
+        account = Fabricate.build(:account, domain: 'domain', username: 'the doctor')
+        account.valid?
         expect(account).to model_have_error_on_field(:username)
       end