Meeting end dates shouldn't be allowed to be set before the start dates

[ilieandriuta]

STEPS TO REPRODUCE

1. Create a meeting by selecting a certain Start Date and time (e.g. 10/06/2020 15:53)
2. As End Date, select a date from the past (before the Start Date, e.g. 05/04/2020 16:00)
3. Click 'Save & View'

EXPECTED RESULTS

The meeting entry cannot be saved (eventually a warning message is displayed, informing that the End Date cannot be set before the Start Date).

ACTUAL RESULTS

The meeting is saved nevertheless and with a wrong duration (got -53 min):

Environment: Windows 10 Pro 64bit, Firefox 77, using an instance of XWiki 11.10.5 on Oracle 19c, Tomcat 9.0.35