This commit will allow plugin developers to enable/disable the custom homepage.
Usage:
```ruby
register_modifier(:custom_homepage_enabled) do |enabled, args|
true
end
```
Args might contain request and/or current_user.
This ensures that the theme id is resolved as early as possible in the
request cycle. This is necessary for the custom homepage to skip
preloading the wrong data.
Joffrey JAFFEUX
Penar Musaraj