[ticket/12254] Better language switching on registration page

PHPBB3-12254
[ticket/12254] Stop using deprecated request_var in user class

PHPBB3-12254
[ticket/12254] Use new set_lang var to avoid conflict with change_lang

PHPBB3-12254
[ticket/12254] Add set_lang var changes to subsilver2 files

PHPBB3-12254
[ticket/12254] Persist set_lang after reg only if changed from default

PHPBB3-12254
[ticket/12254] Use 'language' instead of 'set_lang' as the variable name

PHPBB3-12254
[ticket/12254] No longer need to add change_lang to the URL params

PHPBB3-12254
[ticket/12254] Use a cookie instead of SID to store language selection

PHPBB3-12254
[ticket/12254] Add subsilver2 support for cookie storage instead of SID

PHPBB3-12254
Merge remote-tracking branch 'github-vse/ticket/12254' into develop-ascraeus

* github-vse/ticket/12254:
  [ticket/12254] Add subsilver2 support for cookie storage instead of SID
  [ticket/12254] Use a cookie instead of SID to store language selection
  [ticket/12254] No longer need to add change_lang to the URL params
  [ticket/12254] Use 'language' instead of 'set_lang' as the variable name
  [ticket/12254] Persist set_lang after reg only if changed from default
  [ticket/12254] Add set_lang var changes to subsilver2 files
  [ticket/12254] Use new set_lang var to avoid conflict with change_lang
  [ticket/12254] Stop using deprecated request_var in user class
  [ticket/12254] Better language switching on registration page