Fix infinite loop if user undefined

wf:redirect never get flushed because the request would never end, and
eventually all the wf:redirect wirings would eat up all RAM and crash
the VM
Don't crash if mnesia tables already exist

Like if they were set up but nitrogen was exited or computer rebooted
before the user stuff was all set up.
Fix my indentation
Merge branch 'master' of git://github.com/choptastic/RISE-nitrogen-site into choptastic

Conflicts:
	src/common.erl
	src/db.erl