Translate Plinth web interface into user's preferred language

Translation of Plinth Web Interface is achieved with the help of LocaleMiddleWare. For detailed information on how we achieved, please check https://docs.djangoproject.com/en/1.10/topics/i18n/translation/#how-django-discovers-language-preference.

Summary of PR:

• For Anonymous User: Form to select their preferred language.
• For Known User
  • Creation of new model named UserProfile to accommodate the user's attributes not supported by django's default User model. This design decision is based on the documenation https://docs.djangoproject.com/en/2.0/topics/auth/customizing/#extending-django-s-default-user. For now the created UserProfile have preferred_language attribute to track a known user's language preference.
  • Updatation of user create form, user update form to accommodate preferred language selection.
• Set necessary variables (as described in the how-django-discovers-language-preference) to translate web interface