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
Merge branch 'master' into mkdown-cheat-sheet-link
Added markdown link to new thread view #236

Added markdown help link to new thread view (added it to new post view
in previous commit). Refactored CSS by moving relevant rules to from
thread to post.css.scss, and placed rules in scope of new post and new
thread markdown. Changed the text from "Parsed as Markdown" to
"Markdown cheat sheet"