Omnibus support for two versions of GitLab Pages

Description

In order to roll out the architecture changes to GitLab Pages it will be necessary to support two versions of Pages for a period of time. This can be done by support two version of Pages in Omnibus and allowing that version to be toggled via feature flag or env variable.

Proposal

This work has already started in this MR which can be used as a starting point for this work.

Links / references

Related Epic: &1316 (closed)