Add admin functionality for Contributor(Role)s

Fixes #484
Tidy flatpages/layout templates
Tidy pages dashboard templates
Move reviews templates into catalogue folder
Add Transifex config and package
Add English translation files

Required by Transifex as the source language
Update .po files

This was done by Transifex command.
Add Transifex helper make command
Add Transifex link to contributing section