Diff with better colors and some spacing on the corners

fixed broken gitlab version determination during backup

Upgrade guide cleanup

move the update guides from the wiki into the repo

move the update instructions for 4.2 to 5.0 from the Wiki into the Repo

Instructions update: install postfix separately, new privilege to mysql users, sockets dir. Fixes #3639

Fixed typo in app/views/deploy_keys/index.html.haml

Handle several RDBMS