Ticket/12342 Javascript Bugs and Fixes

http://tracker.phpbb.com/browse/PHPBB3-12342

1. Use prop() instead of attr() when checking/setting boolean properties

2. Fix the Find a member/Hide a member ajax search form

3. Fix the custom date script in UCP Global Prefs. When a user has
chosen a custom date, any time the UCP Global Prefs page loads,
it should reveal to the user the normally hidden custom date field.
This function was being triggered at the wrong time, so it was trying
to use functions that did not exists yet.

PHPBB3-12342