[ticket/11303] add S_TZ_PRESELECT template var

add a new template variable to check if timezone has already been selected
by the user while registeration.

PHPBB3-11303
[ticket/11303] use S_TZ_PRESELECT in subsilver

PHPBB3-11303
[ticket/11303] remove additional brackets

PHPBB3-11303
[ticket/11303] rename attribute to timezone-preselect

PHPBB3-11303
Merge remote-tracking branch 'remotes/dhruv/ticket/11303' into develop

# By Dhruv
# Via Dhruv
* remotes/dhruv/ticket/11303:
  [ticket/11303] rename attribute to timezone-preselect
  [ticket/11303] remove additional brackets
  [ticket/11303] use S_TZ_PRESELECT in subsilver
  [ticket/11303] add S_TZ_PRESELECT template var