refactor: use shared DateSelectedValidation enum instead of DATE_VALIDATION_OPTIONS

[yong-jie]

Problem

An enum called DateSelectedValidation was introduced during a typescript migration task #749, but the frontend codebase has not been updated to make use of this enum.

Closes #777

Solution

Remove the naming conflict between DateValidationOptions and DATE_VALIDATION_OPTIONS. The former is actually a list of options and the latter is the enum. The former is now called DateValidationOptionList.

Edit usages of the enum from lower-case to title-case to match the new enum. i.e. DateValidationOptions.custom to DateValidationOptions.Custom

Reviewers

@tshuli appreciate if you could take a quick look to see if these changes are indeed what you had intended.

[karrui]

lgtm, but wait for @tshuli for final say hehe

[tshuli]

thanks! lgtm :)