Multiple language support of login page

[anatawa12]

See Also: #4480. findBestLocale function in this PR is very similar to findBestLocaleFor in #4480. If both is going to be merged, They may be merged.

In this implementation,

1. For /login
   1. If locale namespaces are enabled
      1. If client sends accept-language which has installed locale
         1. Redirected to /<locale by accept-language>/login
      2. Otherwice
         1. Redirected to /<default locale>/login
   2. Otherwice
      1. Login form will with default locale be rendered as before
2. For /<locale>/login
   1. Login form with specified local will be rendered.