Subforum subscription settings

Settings page now lists the different subforums and allows to subscribe/unsubscribe to them from one place.
This is less elegant than it can be. (My fault as I wrote this code with you.)

Instead of

```ruby
json.subforum_groups do
  json.array! query do |group|
    ...
```

we can do

```ruby
json.subforum_groups query do |group|
  ...
```

There's another place below in this file where we can do the same thing.