Separate date and datetime options in date picker control

[GeoSander]

Description

This PR breaks the "Date or date time" option in the DatePickerDirective.js into 2 separate options:

• Date (only)
• Date and time

Date (only) will be the default option when creating new records.
For existing records, if there's a time component (and hideTimeInCalendar is not set), it will show date and time first.

Furthermore, the directive has been cleaned up and refactored a bit, and the CSS simplified (using flex). The HTML dropdown contains an additional option (if hideTimeInCalendar is falsy) and 2 new translation keys (dateOnlyMode and dateTimeMode) so there will not be any conflicts with the old dropdown translations for older versions.
The calendar icon for the mode selector was changed to a cog icon to make clear that it's a config option and to make sure that it cannot be confused for the other calendar icon in the date field.

Please read issue #9152 for more context.

Checklist

• I have read the contribution guidelines
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• Good housekeeping of code, cleaning up comments, tests, and documentation
• Clean commit history broken into understandable chucks, avoiding big commits with hundreds of files, cautious of reformatting and whitespace changes
• Clean commit messages, longer verbose messages are encouraged
• API Changes are identified in commit messages
• Testing provided for features or enhancements using [automatic tests](https://github.com/geonetwork/core-geonetwork/blob/main/software_development/TESTING.md)
• User documentation provided for new features or enhancements in manual
• Build documentation provided for development instructions in README.md files
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[josegar74]

@GeoSander I have done this test:

1. Create a new ISO19139 metadata, and set a date.

2. Save the metadata --> the form shows the date properly

3. Change to datetime and set the time.

4. Save the metadata --> the form shows the datetime properly

5. Change to date and save the metadata --> the form shows the datetime instead of the date

[GeoSander]

Thanks for the review @josegar74, I fixed ES5 compatibility (and adjusted my IDE to skip ES6 inspections).
I will also do some more tests now switching between modes and saving the metadata, to see if I can make that more robust.

[GeoSander]

@tylerjmchugh Are you able to have a look at this?

[josegar74]

Thanks @GeoSander, the issue described is fixed and looks working well.

Nice improvements have also been made to the code, including for managing time zones.

[tylerjmchugh]

Tested, works as expected.

• Time is hidden by default with the option of adding a time (unless hideTimeInCalendar)
• Time is shown if the existing data includes a time.