Fix #494 convert CI time zones in user_time()

- Added a standard_timezone() function to MY_date_helper to return a PHP
time zone when given a Code Igniter time zone string.
- Used standard_timezone() to convert $current_user->timezone in
user_time() (still assumes we have a valid $timezone if it doesn't go
into the if ( ! $timezone) block)
Fix #827 Sysinfo display of Server vs. Local time

- Modified Sysinfo to use user_time() from MY_date_helper to display the
local time
- Used the format string support in PHP's date() function to format the
server time (as opposed to the manual formatting that was used
previously)
- Used a single format string (assigned to a variable) to ensure both
times are formatted the same way