[BUGFIX] Make date/time fields working again

This patch solves an issue with time-fields and a couple of
inconsistencies for time(sec), date and datetime fields in BE:

- BackendUtility::time() is superfluous as gmdate() can be used
- Ensure stored data is identical to TYPO3 7
- Only use ISO dates in UTC timezone to communicate with JS
  (time fields use 1970-01-01 as date)
- Only write ISO dates in UTC back to hidden FormEngine fields
- Do that for all date/time fields to simplify JS code
- Take care of necessary timezone conversion for date(time) fields
- Correctly handle manual input
- Fix record titles if a date field is the label field
- Fix wrong Acceptance tests
- Fix wrong Unit tests
- Remove superfluous data-date-offset attribute for DateTimePicker
- Streamline FormEngineValidation date handling code
- Refactor DateTimePicker: fix code duplication, work in UTC only

Resolves: #79249
Releases: master, 8.7
Change-Id: I12ef6b6f59f0843182e093f8f274989161820894
Reviewed-on: https://review.typo3.org/51242


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Johannes Kasberger <johannes.kasberger@reelworx.at>
Tested-by: Johannes Kasberger <johannes.kasberger@reelworx.at>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>