Add support for a separate admin language

[rasmusbe]

I would like to suggest a change to use separate language switchers for the site and the admin pages.

Use case:
An english speaking administator is managing a swedish site. The content is created by an swedish speaking translator, but the english speaking administrator also has to use the site. Right now both the site and the admin pages has to be in swedish which makes it hard for the administrator to use the interface.

Proposed changes:

• The admin interface uses a function get_admin_locale instead of get_locale. get_admin_locale gets its setting from an option with key ADMINLANG, but falls back to WPLANG option if missing.
• A filter admin_locale is used in the get_admin_locale function to support extended functionality. (For example user specific settings)

[ocean90]

Duplicate of #29783.