View source for IntlDateFormatter

Implement the ##IntlDateFormatter::format()## for our date and time formatting as well let the IntlDateFormatter determine the DST according the chosen timezone.

  * ((https://unicode-org.github.io/icu/userguide/format_parse/datetime/ ICU Dates and Times Format))
  * ((https://www.php.net/manual/en/datetime.format.php DateTime::format))

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===Timezone===
**Upgrade**: Set timezone for all users back to ##UTC##, the user must set his timezone again. The current offset is not unique and the DST depends on the location.

**Registration**: The Browser's Timezone can be detected via JavaScript, however this is also guesswork. Should we set here ##UTC## as default too?.

===Upgrade===
This changes a few things:

====New format syntax====
The installer resets your custom settings to the defaults. Change it again via the Admin panel with the new syntax.

#|
*| config | DateTime::format | IntlDateFormatter |*
|| date_format | ##d.m.Y## | ##dd.MM.yyyy## ||
|| time_format | ##H:i## | ##HH:mm## ||
|| time_format_seconds | ##H:i:s## | ##HH:mm:ss## ||
||  |  |  ||
|#

For example you might want set the ##date_format## to ##dd/MM/yyyy##.

ICU Formats
((https://unicode-org.github.io/icu/userguide/format_parse/datetime/#date-field-symbol-table Date Field Symbol Table))

%%
date_format($unix_time, $pattern)
%%
- the unix time must be passed as UTC time

====Timezone====
The installer sets the timezone for the system as well as for all users back to ##UTC## as default. Because the timezone DST is determined by the location the users have to set their location in the user settings again, the timezone offset is only an indicator and not unique.

If all of your users work in one place, you can set the timezone via a SQL query:
%%(hl sql)
UPDATE wacko_user_setting SET timezone = 'Pacific/Tahiti' WHERE timezone = 'UTC';
%%

====Installation====
Do not forget to delete per hand the config cache and restart your browser before you run the upgrade process, so you get a new session - else it will use your old session after the upgrade and you will get a 500er error, because the 'timezone' must be set 'UTC' in the session.

This change will be available with WackoWiki **6.1.4** release.