Guideline for passing single objects from backend to frontend (current user, current project, ...)

For https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13866 there was a Slack discussion what would be the preferred way to pass single objects (not lists) from backend to frontend. We currently have multiple ways to do this in our code base:

• global (property of window) (we want to reduce that: https://gitlab.com/gitlab-org/gitlab-ce/issues/20983)
• part of gon (we want to reduce that, couldn't find the issue)
• as service / singleton + Ajax call (https://gitlab.com/gitlab-org/gitlab-ce/issues/31953)
• via Vuex + Ajax call (https://gitlab.com/gitlab-org/gitlab-ce/issues/33985)
• data attributes
• <script id="..." type="application/json"> and JSON.parse(document.getElementById('...').innerHTML)

Ajax calls can be cached in localstorage to increase performance.