guidelines for server rendering vue components

Description

Currently vue components are rendered to replace empty divs, so UI jumps around a lot.

Proposal

consistent way of rendering Vue components from HAML

could be one of:

• show loading spinner / placeholder [as seen in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11434]
• duplicate content in HAML, or show stripped-down readonly version
• actually server render
• unserver render - hide the entire UI until JS has loaded then show everything

Choice should be added to https://gitlab.com/gitlab-org/gitlab-ce/issues/31580

Links / references

Vue has server renderer but needs node :(