Vue.js timeago: add missing functionalities

We are missing some functionalities in vue.js instances of timeago.

We need to port this https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/javascripts/lib/utils/datetime_utility.js

into a vue.js mixin that can be used between several components. That mixin already exists: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/javascripts/vue_shared/mixins/timeago.js