Minor changes to installation and databases docs.

Created by: axilleas

• I added relative urls to various references of other documentation files in installation.md. Also there were some duplicate commands regarding the update of a Debian system.
• databases.md is also fixed to depict the recent changes of the user used by Gitlab. As of 5.0 it is git and not gitlab.
• For 46f7b7c1 see https://groups.google.com/forum/?fromgroups=#!topic/gitlabhq/-4JLrOrpaGM