More explicit dev setup instructions

[ekohl]

This adds explicit Fedora and Enterprise Linux instructions. It also adds a section on PostgreSQL. This makes the alternative database.yml no longer needed. It also sets and explicit admin password instead of relying on the auto generated value.

Also corrects the minimum PostgreSQL version.

[stejskalleos]

@nofaralfasi review please

[stejskalleos]

Merging is blocked by missing required checks