Autosubscribe to new threads and unsubscribe links in email

There's now a new user setting: "Subscribe me to new threads created in subforums I'm subscribed to." This will default to true. Since this means more email volume by default, this pull request also includes support for unsubscribing from threads or subforums using a link sent in the email.
This generates some ambiguous route helpers. On line 12, we have rails generate `thread_url`. Normally, `resources :threads` would also do that, but it seems rails detects that `thread_url` is already defined and doesn't redefine it here. That said, if you move this block above line 12, we get the following error:

```
/Users/david/.rvm/gems/ruby-2.1.2/gems/actionpack-4.1.4/lib/action_dispatch/routing/route_set.rb:430:in `add_route': Invalid route name, already in use: 'thread' 
You may have defined two routes with the same name using the `:as` option, or you may be overriding a route already defined by a resource with the same naming. For the latter, you can restrict the routes created with `resources` as explained here: 
http://guides.rubyonrails.org/routing.html#restricting-the-routes-created
```

This ambiguity is not ideal (it confused me), but I'm not sure exactly what we should do about it. One thing we could do is not use `resources` and instead do this:

```ruby
get '/threads/:id/unsubscribe' => 'threads#unsubscribe', as: 'unsubscribe_thread'
```

But I'm not sure if this is great.