Database strategy and code compatibility between CE, EE and JH

It's key to business opportunities of both JH and GitLab to ensure full code & database compatibility between our versions.

This issue is created out of the need to have a strategy at a high-level. It is essential that we continue to support the switch back/forth between the versions.

Upgrade paths

From our path forward in #161 (closed) regardless of options we choose (split or no-split schema) we need to ensure the following paths are compatible:

• Upgrade from CE
  • to EE
  • to JH
• Upgrade from EE
  • to CE
  • to JH
• Upgrade from JH
  • to CE
  • to EE

Decision

We are going with the unified approach gitlab-com/www-gitlab-com!90048 (merged)

@qianzhangxa @jeromezng @craig-gomes @cdu1 @kwiebers @tpazitny @ben_lin