Organize date utility functions

What does this MR do and why?

Organize date utility functions

• Move isValidDateString from its own file to date_calculation_utility.js
• Move tests from datetime_utility_spec.js to either date_calculation_utility_spec.js or date_format_utility_spec.js.

datetime_utility.js just imports other files:

export * from './datetime/date_calculation_utility';
export * from './datetime/date_format_utility';
export * from './datetime/locale_dateformat';
export * from './datetime/timeago_utility';

Since datetime_utility.js doesn't contain any utility functions itself so it's confusing for it to have a spec file, hence tests in datetime_utility_spec.js are moved to more appropriate spec files

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