ManageSieve: make sieve_formattime() parse all time-formatters PHP does.

[bennet0496]

Roundcube's $config['time_format'] understands all PHP time and especially timezone format specifiers listed in the PHP docs; however, sieve_formattime() that used this value via rcmail.env.time_format does not.

Therefore if the config value e.g. is $config['time_format'] = "H:i T" to include the abbreviated timezone name (which works else where, like the message list), then forms using sieve_formattime(), the OOO form, will generate times like "6:00 T" and so on

This causes the actual time be set to "Tango Time", i.e. UTC-7

Which for users outside of UTC-7 is unexpected behavior.

Therefore I changed sieve_formattime() to understand any formatter that may reasonably (or unreasonably, like microseconds or Unix Time. But never say never) be configured by an admin or user. For the timezone values, I opted to pass these values from PHP to JS via set_env() as in JS these values are extraordinarily hard to reliably to obtain without additional libraries.

I also changed some thing at the beginning of sieve_formattime() that looked odd to me. Like per-declaring all variables. And actually used the value that h would have been in the g and h cases. But I'm also happy reverting these changes if they are undesired.