Add a user preference to override implicit time formatting based on
locale to be either 12 or 24-hour/iso time format. This MR is based on
https://gitlab.com/gitlab-org/gitlab/-/issues/15206.

This is automatically applied anywhere `getTimeago` from
`app/assets/javascripts/lib/utils/datetime/timeago_utility.js` is being
used. This results in a number of places currently using hardcoded time
format strings being uneffected by the setting.

Changelog: added