default_value_for :preferred_language is not set with the config.i18n.default_locale value
### Summary
Gitlab User model doesn't set preferred_language attribute from config.i18n.default_locale value.
GitLab: 12.0.3 (gitlab-ce@08a51a9db938bb05f9a4c999075d010079e16bad)
GitLab Shell: 9.3.0
PostgreSQL: 10.7
### Steps to reproduce
1. Set a default_locale in application.rb
config.i18n.default_locale = :fr
config.i18n.enforce_available_locales = false
2. Create a new user
3. User preferred_language attribute value is always English
### What is the current correct behavior?
User default preferred_language attribute value should reflect the default locale
Current :
```ruby
# with config.i18n.default_locale = :fr
default_value_for :preferred_language, I18n.default_locale
# preferred_language value is always :en
```
Expected:
```ruby
default_value_for(:preferred_language) { I18n.default_locale } # bug fix
# preferred_language value is now :fr
```
issue