Add settings finder method on model class

I got tired of doing this in the console all the time

```ruby
User.find_by_settings("any.old.setting")
# => [users that have that setting and it's truthy]
# User.find_by_settings("any.old.setting" => true) is equivalent

User.find_by_settings("any.setting" => "blah")
# => [users with "any.setting" => "blah"]

User.find_by_settings("any.setting" => false)
# => [users that don't have that setting or it's falsey]

User.find_by_settings("setting" => "foo", "other_setting" => "bar")
# => [users that match all conditions]
```