Support most Gitlab config customisation

Currently we are missing the ability to configure the following:

• no relative_url
• no custom trusted proxies !260 (merged)
• UTC only timezone !260 (merged)
• no cron job customization !283 (merged)
• This means no ldap !278 (merged)
• No omniauth settings #335 (closed)
• No google analytics !260 (merged)
• No rack-attack configuration options !260 (merged)