Introduce DateHelper->format methods

The formatDate/formatTimeDateformatDateTime of DateHelper should be user
in place of the constants.
Fix date/time format string
Try fixing Travis CI error by updating translations
Formatting PHPDoc
if/else in stantard psr-2
Allow customization of date/time formats
Force initial locale of tests to en_US
Separate time-zone and translations tests
Add tests for new DateHelper::format methods
Deprecate special constant-based date/time formats
Deprecate DATE_APP_DASHBOARD_SEARCH_RESULTS_PAGES

Some constants removed
Enable custom-defined timezone in date formatting
Print date-only attributes in system timezone
Bugfix
Fix formatting survays results date/time

Let's use the new DateHelper::format function (correctly handles timezone)
Run tests at system level

Avoid potential problems due to timezone associated to the current user
Add formatPrettyDate/Time methods to DateHelper
Fix double converting to user timezone

Both getLocalDateTime and formatCustom handles timezone.
Let's avoid double timezone assignements.
State that default timezone is the system one

When converting string representation of date/time, let's say that the
DateHelper::toZendDate assumes that the source timezone is the system one.
Bugfix