When editing existing encounter, time component of encounter datetime should not be lost

When editing an existing encounter via form with a date (but not a time) widget, we don't want to lose the time component of the encounter datetime if the date of the encounter is not changed.

The main use case of this is when editing an encounter that is created POC and/or real time... we don't want to lose the timestamp.  And it will case a validation error if it ends up falling outside of the visit

This was initially implemented in the HTML Form Entry UI module here:

https://github.com/openmrs/openmrs-module-htmlformentryui/blob/master/omod/src/main/java/org/openmrs/module/htmlformentryui/fragment/controller/htmlform/EnterHtmlFormFragmentController.java#L242

However, after upgrading to Core 2.2 we are seeing visit validation errors so it seems this needs to happen earlier in the process, so we are moving this functionality into HTML Form Entry.