This adds some pre-reset checks to rbdemo's reset-demo command that

ensures we have all the proper permissions and abilities we need prior

to resetting the demo database state. This includes read-access to the

fixtures and media directories, write access to the destination media

directory, and the ability to change ownership. It also validates that

the specified user and group are valid.

Tested each possible validation failure and saw that I got appropriate

errors.
Tested with a valid setup and saw it reset the demo database.