Add "scrubbed backups" capability

Adds a --scrubbed-backup flag for backup generation for development
use.  This generates backups with all user-identifying and
authentication information (email addresses, passwords, password reset
nonces) removed.  Email addresses are replaced with a non-existent
address, and all user accounts in the backup are given the password
"test".

You can use a scrubbed backup to restore into a database that you can
interact with just like the main Hackage database.  This is a more
effective means of making a development test database than mirroring
individual packages from the main Hackage server since it reproduces
all package download statistics from the main server, along with all
user accounts and other information from the main server.
Make HTML validation test safe (sort of fixes #227)

Prevent failure to connect to html5.validator.nu HTML validation
service from causing test failures.
Merge pull request #228 from ian-ross/safe-html-validation

Make HTML validation test safe (addresses some of #227)
Remove debug code accidentally left in
Merge pull request #223 from ian-ross/scrubbed-backups

Add "scrubbed backups" capability
Merge branch 'master' into central-server