main: refactor locale-related changes:
main: refactor locale-related changes:

- rename self.needed_sso_vars -> self.needed_session_vars to be clear
- add 'configured_locale' to needed_session_vars for future use
- rename get_client_language() -> parse_client_accept_language()
- add get_selected_locale_code() to determine locale code to use in current request
- add gettext() helper to translate in py code using current request's locale
- call self.setup_locale() in setup_template_vars()

Template now knows following additional vars:
- LOCALE: 2-letter locate code for current template generation
- tr: translator object to use for translations inside templates.
  Use tr.gettext() to translate strings inside templates, e.g:
  ${tr.gettext('test')}

minlexx/whdbx_web,minlexx/whdbx_web,minlexx/whdbx_web,minlexx/whdbx_web,minlexx/whdbx_web