Minor changes to installation and databases docs.

* 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 46f7b7c1f2be9ecbc2d81c886212dcd81629700e see https://groups.google.com/forum/?fromgroups=#!topic/gitlabhq/-4JLrOrpaGM