Fix existing subforum auto-subscription.

There were two bugs here:

1. We were trying to subscribe users to subforums before creating the
user. This means the subscriptions didn't have a user.
2. Subscriptions weren't validating the presence of the user and the
subscribable thing.

This commit fixes both of these issues.
Auto-subscribe new HSers to Welcome and Housing
Subforums should have unique names.

Zach and I already agreed on this in person, but we never put it in
code. Today during development, I ended up with five subforums with each
name. I can't reproduce that issue any more, but having this validation
(and associated change in seed data), will make sure that the next time
something wonky like this happens, we'll know about it.
Seperate account creation logic.
Subforum names are now case-insensitive unique