Add time preferences for user [FE]

Description

We'd like to make displayed times in GitLab more customizable:

• Set timezone
• Change timezone to 24h
• Absolute times vs. relative

In order to enable these changes, we need to support these settings on the backend. We're doing this in https://gitlab.com/gitlab-org/gitlab-ce/issues/57620.

Proposal

Once these attributes are available, we should allow them to be set in the user profile and user preferences. UX needed:

• Timezone: consider making this available as a dropdown in the user profile. See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/999 for an example.

• Time display preferences (12h/24h, absolute/relative): consider displaying in the profile section or in Preferences.

We should also implement these datetime modifications, as investigated in https://gitlab.com/gitlab-org/gitlab-ce/issues/56997.