Now using the input type date and time to set start and end dates/times on sales; input field width formatting

[kimcoleman]

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Changes proposed in this Pull Request:

The HTML input types "date" and "time" are now widely supported across browsers and should be a better user interface for setting sale start and end dates / times.

This PR swaps to using the single input types and updates how the meta is saved to string the date input and time input into one stored meta value.

Other information:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] Have you successfully run tests with your changes locally?

Changelog entry

• ENHANCEMENT: Now using date and time HTML input types for sale start and end date/time settings.

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.