Audited the dateformat usage on custom emoji page

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What does this MR do and why?

Refactor date formatting in custom emojis List component to use locale_dateformat utility

• Updated date formatting in app/assets/javascripts/custom_emoji/components/list.vue to utilize locale_dateformat from app/assets/javascripts/lib/utils/datetime/locale_dateformat.js.

Related to issue #463549 (closed)

Audited the dateformat usage on custom emoji page while creating new custom emojis

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