Replace database converter

[dosire]

Replace https://github.com/gitlabhq/gitlab-recipes/tree/master/database/migrate-mysql-to-postgres with a link to the official one https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/update/mysql-to-postgresql.md

[axilleas]

Should we better merge the guide in recipes to the one in gitlab repo?
I have merged it here
What do you think?

[dosire]

@axilleas I think the merged version is trying to do to much. The following initial setup was taken from installation.md from the main installtion doc => we should not copy stuff, keep it dry. The debian dependency in the beginning is also not needed, it should work on most platforms. I think the official one can be made pretty and have hints added, but it should not try to do more.