Update Existing Documentation as of 2017-12-14

Update a large swath of documentation for recent changes.

• Move Omnibus chart README under doc/gitlab/omnibus to align with chart hierarchy
• Update Omnibus chart README with extensive additions for individual settings introduced as we split components out.
• Update doc/example-config.yaml to a full in-chart deployment, with all new charts deployed where possible.
• Tweak doc/external-db with formatting and link to doc/example-config.yaml

This does not add any additional documentation pages, such as for Unicorn (#86 (closed)) or Shell (#92 (closed)).