Permit dots in usernames with conditions (#6844)

* Permit dots in usernames with conditions

- Dot cannot be the start or end of username
- a.lice and al.ice are considered the same during sign-up

* Fix regex mixin flags

app/validators/unique_username_validator.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class UniqueUsernameValidator < ActiveModel::Validator
+  def validate(account)
+    return if account.username.nil?
+
+    normalized_username = account.username.downcase.delete('.')
+
+    scope = Account.where(domain: nil, username: normalized_username)
+    scope = scope.where.not(id: account.id) if account.persisted?
+
+    account.errors.add(:username, :taken) if scope.exists?
+  end
+end